An interesting incident is quoted in Tafseer al-Mizan from the book Amali. Imrul Qays and another man had a dispute regarding some property. Both of them came to the Holy Prophet (s.a.w.s.). He (s.a.w.s.) asked Imrul Qays,
"Can you provide two just witnesses to substantiate your claim?"
He replied, "No!"
The Holy Prophet (s.a.w.s.) said,
"Then your opponent should take an oath."
Imrul Qays said, "But what if he swears falsely and acquires my property?"
The Holy Prophet (s.a.w.s.) replied,
"If he swears falsely he shall be included among people who will not be eligible for Divine Mercy on the Day of Judgment and Allah shall not purify him of sins. There would be a dreadful punishment for such a man!"
When the litigant heard these statements he was filled with horror and gave up his false claim to the property of Imrul Qays.
Hazrat Imam Ja'far as-Sadiq (a.s.) is reported to have said:
"One who knowingly takes a false oath had made war upon Allah."
(Al Kafi)
Imam Muhammad al-Baqir (a.s.) says that the Holy Prophet (s.a.w.s.) said:
"Refrain from false oath, because it destroys inhabitations and makes the sinner helpless."
(Al Kafi)
According to other traditions, false oath and severing relations are two such sins that cause the destruction of towns and cities. The inhabitants are eliminated and the progeny terminated.
Evil consequences of false oaths
Hazrat Imam Ja'far as-Sadiq (a.s.) says,
"One who swears falsely becomes poor within forty nights" (means forty days).
(Al Kafi)
Many traditions of similar connotations are available. The same Imam (a.s.) has also said:
"The false oath that takes one to the fire is the one which is taken to usurp the right of a Muslim or to usurp his property."
And Imam Ja'far as-Sadiq (a.s.) also remarks,
"When a person says, "Allah knows!" when in fact he is speaking a lie; then Allah tells him, 'Didn't you find anyone other than Me to ascribe the falsehood to?'
(Al Kafi)
The Holy Imam (a.s.) also says:
"When a person says 'Allah knows' and the fact is that Allah knows contrary (to what he alludes), the heavens shudder due to the Might and Divine anger."
Types of oaths
An oath is taken to prove some fact or report, or to relate the same with emphasis. There are four kinds of oaths:
1. Wajib (Obligatory).
2. Mustahab (Recommended).
3. Makrooh (Detestable).
4. Haraam (Prohibited).
When does it become wajib (obligatory) to swear?
It is wajib to take an oath in a situation where one's life or honour, or that of another Muslim, is in danger, and taking the oath can ward off the danger. When it is wajib to protect ones property it is also wajib to take an oath for its protection. In fact in all the above situations it is wajib even to take a false oath, although as a precautionary measure one should first try ones best to employ Toriya.
Mustahab oath
There are situations where it is Mustahab to take an oath or to refrain from doing so. In case of very insignificant property belonging to oneself or to another Muslim it is not wajib to swear. In this case it will be Mustahab to do so. Property that is usually considered insignificant is worth thirty Dirhams or less.
Zurara asked Imam Muhammad al-Baqir (a.s.): "The oppressive rulers forcefully collect taxes from us. Can we swear falsely that we have nothing to give tax on when it is not possible to save our money without swearing falsely?"
Imam (a.s.) said,
"Take such oaths! Such oaths are more sweet than dates and butter."
(Wasaelush Shia)
However if the property is not worth much, especially if its value is less than thirty Dirhams, then it is Mustahab not to swear falsely, even if it may be necessary to save oneself from the oppressor.
Hazrat Imam Ja'far as-Sadiq (a.s.) has quoted the following tradition from the Holy Prophet (s.a.w.s.):
"Respecting the Greatness of Allah if one refrains from swearing, Allah shall give him much better than whatever he has lost."
(Furu Al Kafi)
Imam Ja'far as-Sadiq (a.s.) says:
"If there is claim against you regarding some property and the claimant does not have any right upon you, and desires to make you take an oath, then if the claim is for something worth less than thirty Dirhams, hand it over to him and do not take an oath. But if it is worth more than thirty Dirhams, take the oath and do not give him anything."
(Furu Al Kafi)
The Holy Prophet (s.a.w.s.) says:
"If a person takes his debtor (or one who owes him something) to the court of the ruler; and the king asks him to swear; and knowing that he is on the right he respects the Greatness of Allah and refrains from swearing, then on the Day of Qiyamat Allah shall not desire for him a position less than that of Hazrat Ibrahim (a.s.)."
Imam Sajjad (a.s.) avoids swearing
In the book Al Kafi there is a tradition, which says that a wife of Imam Sajjad (a.s.) had some connection with the tribe of Bani Hanifa. A Shia of Imam (a.s.) informed him that this wife of his bore enmity to Amirul Momineen (a.s.). After investigating the matter Imam (a.s.) divorced her. She had already received the Meher amount (Dower), but she filed a claim for it against Imam (a.s.) in the court of the ruler of Madinah. She demanded four hundred Dirhams as Meher from him. The ruler of Madinah told Imam Sajjad (a.s.), "Either you swear that you have already paid her or you pay the amount of Meher (now)." Hazrat Syed-e-Sajjad (a.s.) did not swear, but ordered his son Hazrat Muhammad al-Baqir (a.s.) to pay four hundred Dirhams to the woman. Hazrat Baqir (a.s.) said, "May I be sacrificed for you! Are you not on the right?"
Imam (a.s.) replied,
"Why not? But I consider Allah to be much more honourable than this matter, that I swear by His name for some petty worldly property."
Swearing for the sake of emphasis
Not only it is permitted but it is mustahab to swear in order to emphasize on a true fact, or to prove some fact, or to show its importance. For example the Holy Prophet (s.a.w.s.) speaks after taking the oath:
"By Allah! Allah does not delay in forgiving, Though you may be lazy in seeking it."
(Masaalik)
Another example of an oath is the following saying of Amirul Momineen (a.s.):
"By Allah! If people knew what I know then very few would have laughed and many more would have wept."
(Masaalik)
There are numerous Quranic verses and the traditions of Masoomeen (a.s.) where oath is taken. All of them are of this same type. They are for emphasis and for proving particular facts.
A person wrote a letter to Imam Muhammad Baqir (a.s.) and asked about something which was being wrongly attributed to him (Imam a.s).
Imam Baqir (a.s.) wrote the following reply,
"By Allah! What is being attributed is not correct. But under no circumstance do I like to say "By Allah" to disprove it. Yet I regret that such a thing is being said when it really isn't so."
(Mustadrakul Wasael)
Swearing is Makrooh
We have already described the wajib and Mustahab oaths. Apart from these, in all other situations, it is Makrooh (detestable) to swear. It is irrelevant whether the oath is for something past, present or future. In ordinary situations, for ordinary matters, taking an oath is Makrooh. Swearing for a false thing is certainly Haraam. It is the command of Imam Ja'far as-Sadiq (a.s.):
"Do not swear by the name of Allah, whether you are speaking the truth or lying."
(Furu Al Kafi)
"Because Allah says in the Holy Quran,
"And make not Allah because of your swearing (by Him) an obstacle to your doing good...""
(Surah Baqarah 2:224)
Hazrat Imam Ja'far as-Sadiq (a.s.) also says,
"One who swears falsely by Allah for a lie has disbelieved, and one who swear by Allah for truth has sinned, because Allah says: "Do not use Allah for (taking of) your oaths."
(Furu Al Kafi)
The above quoted traditions could be simply explained thus:
To swear falsely in the name of Allah is definitely a greater sin and one who indulges in a greater sin, falls down from the highest position of Faith. Due to this some amount of disbelief comes to his heart. Imam (a.s.) has also called a true oath a sin and he has used the word "Ithm" (sin). Since it is prominent among the Mujtahids that a true oath is Makrooh the word "Ithm" would imply something which is very very undesirable.
A Lesson from Hazrat Isa (a.s.)
The book Furu Al Kafi also contains the following report from Imam Ja'far as-Sadiq (a.s.):
"The apostles of Hazrat Isa (a.s.) came to him and said, "O the teacher of good, give us some advice."
He (a.s.) said, "Certainly, Musa, the Prophet of Allah commanded you not to swear falsely by Allah. And I order you not to swear falsely and (nor) truly."
(Furu Al Kafi)
Oath of respectable objects and personalities
Wherever it is permitted to swear by Allah, it is also permitted to swear by other honorable personalities or respectable objects. In such situations it is allowed to take an oath by the Quran, the Ka'ba, the Prophet (s.a.w.s.) or the Imam (a.s.). In the same way oath upon any other respectable object is also permissible. For example a person may swear by his father or son etc.
Those traditions that prohibit swearing by Allah's name prohibit it for proving ones right. They do not prohibit the proving of a true fact by oath. However when a situation arises where it becomes obligatory to take an oath, then one must swear only by Allah. Oath by any other object or personality will not be absolutely correct and from the Shariat point of view the matter will not end conclusively. Similarly if one has to take an oath for performing a particular action in the future, in this case too, one must swear only by Allah. Any other vow does not have any legal standing.
The oath that is haraam under all circumstances
The oath which is Haraam under all circmstances and which one can never take is that of dissociating with Allah and His religion. For example a man says;
"If I do not perform this particular action, I shall be dissociated with Allah and His religion." Such an oath is certainly Haraam.
In the same way if one says:
"If I do not do this, I would have disbelieved in the Holy Prophet (s.a.w.s.), or I would have rejected the Mastership of Ali (a.s.), or I would become a disbeliever." Such a vow is also Haraam. It is Haraam whether one wishes to prove the truth or to lay emphasis upon a fact.
The Holy Prophet (s.a.w.s.) heard a person taking such an oath. He (s.a.w.s.) said, "Woe be unto you, if you leave the religion of Muhammad (s.a.w.s.) then which religion would you follow?"
The narrator says that the Holy Prophet (s.a.w.s.) did not speak to this man till the end of his life.
(Al Kafi)
Imam Ja'far as-Sadiq (a.s.) informed Yunus Ibne Zabyan:
"O Yunus! Do not speak about dissociating from us in an oath. One who takes oath from it, whether for a true thing or a falsehood, he really becomes dissociated from us."
(Al Kafi)
source : GREATER SINS by Ayatullah-ul-Uzma Sayed Abdul Husain Dastghaib Shirazi (r.a.)