When is the vow correct?

There are some requirements for the correctness of a vow that a person takes for performing or avoiding particular actions in the future. If all the requirements are fulfilled then it is haraam to break this vow. If it is broken, expiation becomes wajib.

The following are the conditions for such vows:
1. The vow should be with regard to a wajib or a Mustahab act. For example, he can vow that he shall not intentionally avoid the Morning Prayer, he will make it a point to say Namaz-e-Shab. In the same way if one vows to abstain from a particular act, this act has to be either haraam or Makrooh in nature. For example he can vow that he shall not speak a lie in future or he can vow to refrain from spitting in the mosque. A vow to refrain from a Mubah (permitted) act or thing should only be taken when there is some benefit in it. For example it is Mubah to smoke. So one can take a vow in order to give up this habit.

2. Vows with regard to all the five types of actions, i.e. Wajib, Mustahab, Haraam, Makrooh and Mubah are valid only when the name of Allah is uttered with a firm intention to do or to refrain from a particular thing. One must not swear by Allah in jest. If one is in a habit of saying, "By Allah I shall do this." or "By Allah I shall not do it" then unless he seriously means it, such a vow is not considered valid.

The Quran says: 
"Allah does not call you to account for what is vain in your oaths, but he calls you to account for the making of deliberate oaths; so its expiation is feeding of ten poor men out of the middling (food) you feed your families with, or their clothing, or the freeing of a neck; but whosoever cannot find (means) then fasting for three days; this is the expiation of your oaths when you swear; and guard your oaths. Thus does Allah make clear to you His communications, that you may be grateful.
(Surah Maidah 5:89)

Useless Vow

From the foregoing discussion we can conclude that a vow is only valid when there is some inherrent good or evil in the concerned action. Therefore one can take a vow only for acts that one is inclined towards or those that are wajib or mustahab. Similarly if a person vows to refrain from a particular action he must feel an aversion to it or it should be something makrooh or haraam. Hence to vow to perform some lewd action is itself lewdness. Such a vow is invalid from Shariat point of view. We must never vow to perform evil acts whether it is evil according to reason or according to Shariat.

It is haraam for one to vow to omit wajib or mustahab prayers or vow not to speak to ones mother or any other relative. It is also not allowed to vow against performing Hajj if one is capable of doing so. To vow that one would never mediate between two believers is also haraam. All such vows are invalid. Hence if one has ever taken such vows one must repent for them.

Hazrat Imam Ja'far as-Sadiq (a.s.) says,
"If one takes a vow to refrain from a beneficial act he must perform it (and disregard the vow). There is no kaffarah for breaking such a vow. It is only a satanic instigation."
(Al Kafi)

Although apparently the above tradition and other such reports imply that a vow for a mubah act which is beneficial is invalid, the mujtahids are of the opinion that one must exercise caution and in case such a vow is broken, kaffarah should be paid. According to the majority of the jurists a mubah thing becomes wajib if one takes a vow for it. Thus the best thing is to follow the path of precaution.

Saeed Aeraaj, a narrator of traditions says that he asked Imam Ja'far as-Sadiq (a.s.) about a person who had vowed to do a particular thing, whereas the avoidance of that was more beneficial, and now the person himself was worried to disregard his vow. Imam (a.s.) said,
"Have you not heard the Messenger of Allah (s.a.w.s.) say that whenever you find that you have vowed against doing something that is better to be done, you must ignore your vow?"
(Al-Kafi)

Types of Vows

In the same book, Al Kafi we have a tradition of Imam as-Sadiq (a.s.) that he said,
There are three types of Vows:
1. A vow that makes hell incumbent.
2. A vow that makes kaffarah wajib.
3. A vow that neither earns hell nor entails kaffarah.

A vow that makes hell incumbent is a false one. It is a vow that proves harmful to the Muslims. It is called 'Yaman Ghamoos' in the religious terminology.

A vow that entails Kaffarah is one that is taken to perform a charitable act or taken to refrain from an evil act. If a person breaks such a vow he is liable to pay kaffarah for the same. In addition he must feel remorse for it and repent.

Lastly, a vow that neither earns hell nor makes kaffarah wajib is the vow of doing Qat-e-Rehmi taken under duress before a tyrant ruler, parents or ones spouse. Other vows like the doing of some evil act or to refrain from a wajib are also included in this category. Those who wish to study in detail may refer to the comprehensive books of jurisprudence.