Ahmad bin Ishaq Ashari Qummi was the leader of the group of Qom pilgrims who went for the Ziyarat of the Purified Imams (a.s). He is one of the important narrators of traditions. Once a Shia man met him and gave him a letter of Ja’far, the son of Imam Hadi (a.s). In this letter, Ja’far had introduced himself and informed that after the passing away of his brother, Imam Hasan Askari (a.s), he was the leader and chief of the Islamic world. And Ja’far claimed to possess what the people were in need of regarding the lawful and the unlawful matters of the Islamic Law. After he got this letter, Ahmad wrote to His Eminence, the Awaited Imam, and also enclosed the original letter of Ja’far. His Eminence, Imam Mahdi (a.s) replied to Ahmad as follows:
In the name of Allah, the Beneficent, the Merciful
“Your letter has been received, may Allah protect you, and a letter attached to it, with the things mentioned in your letter. In spite of the fact that the words and imports of the letter were metamorphosed and confused and mistakes and errors were numerous, as I have understood; if you had paid close attention to it you would also have realized it.
Praise is only to the Lord of the worlds, a praise not shared by anyone else: Praise be to Allah the Lord of the worlds and by that favor and kindness that is on us from Him. Allah Almighty has perfected the truth and rendered falsehood invalid. And Him that I mention is the witness and there is no doubt in what I am saying, we shall collate these things now. And every fact about which questions shall be raised. And the writer of that letter to you and none from the creatures has the responsibility of Imamate, obedience and covenant. And soon we shall explain to you those matters that shall be sufficient to convince you.
O Ahmad! May Allah have mercy on you. The Almighty Allah has not created people in vain and He has not left them aimless and useless. Rather, by His power He created them and provided them with hearing, sight, mind and conscience. Then He sent towards them prophets, who were warners and givers of glad-tidings. They command the people for obedience of Allah and restrain them from His disobedience. They introduce the divine command and religion to them. He sent down the Book on them and sent angels towards them so that they may be mediums and channels between the Almighty and the prophets.
Among the prophets there is someone for whom Allah made the fire, cool and mercy and made him His friend and confidant.
Among them was one such that Allah spoke with him emphatically and openly made his staff into a python.
Someone from them is such that enlivened the dead and cured the born blind and the leper.
Someone from them is such that Allah taught him the language of the birds and he was given everything.
And after that He sent His Eminence, Muhammad (a.s) the mercy for the worlds. And by him He completed the favor and ended the series of prophets and sent him for all the people of the world. And (he) demonstrated and explained his truthfulness, his signs and his prophethood to the people. At last, he, who was a praiser of Allah Almighty, fortunate and successful, he also met his Lord.
On this point, he entrusted the affairs to his brother, cousin, legatee and inheritor, that is Ali Ibne Abi Talib (a.s) and after that to his successors from the sons of His Eminence one after the other. And revived His religion and perfected His light through them. He accorded them a clear distinction among their brothers, cousins and other family members so that they may be recognized as the proof of Allah and leaders among the common people. In such a way that He made them (Prophet and his legatees) free of all sins and defects and pure from impurity. And also, the Almighty Allah made them the custodians of His knowledge and wisdom and the center of His secrets and confidential matters and supported them with proofs and arguments. And if He had not done so, all the people would have been same from all aspects indeed; truth and falsehood, knowledgeable and ignorant would not have been distinguished.
Therefore, know that this false man has associated falsehood to the Almighty God. And that what he claims (in this letter) is falsehood and fabrication, I don’t know on what he relies to prove his claima On his deep understanding of the religion of Goda By Allah! He is not capable to discriminate between legal and illegal and incorrect from correct. And whether it is on his knowledge and know-howa He is not able to distinguish a fact from a lie and the clear from the ambiguous, and the extent of ritual prayer and its time. Or is it that he relies on his piety and abstemiousnessa Allah is a witness that he omitted obligatory prayer for forty days with the false notion that by omitting prayer he would be able to acquire the art of deception. And may be this information has reached you also. The containers of wine are present for him and signs of his sins and disobedience of Allah Almighty are obvious for everyone. Does he intend to prove his claim by a sign and miraclea Then he should put forward his argument and miracle. Does he rely on any argumenta Then he should mention his argument. The Almighty Allah says:
“Ha Mim. The revelation of the Book is from Allah, the Mighty, the Wise. We did not create the heavens and the earth and what is between them two save with truth and (for) an appointed term; and those who disbelieve turn aside from what they are warned of. Say: Have you considered what you call upon besides Allaha Show me what they have created of the earth, or have they a share in the heavensa Bring me a book before this or traces of knowledge, if you are truthful. And who is in greater error than he who calls besides Allah upon those that will not answer him till the day of resurrection and they are heedless of their calla And when men are gathered together they shall be their enemies, and shall be deniers of their worshipping (them).”
O Ahmad! May Allah give you divine opportunity to test and ask this transgressor about those matters I have explained to you. And ask him about a verse of Quran so that he may explain its meaning and ask him about an obligatory prayer to explain its extent and its obligatory rituals. At that time you would know his condition and limits, understand his defect and ignorance. May Allah make him accountable.
Although the Almighty Allah has not appointed Imamate on any two brothers, except Imams Hasan and Husain (a.s). And in the same way has accorded us permission in some matters. Truth is manifested and falsehood, destroyed. And I am hopeful of the favors and sufficiency. Allah is sufficient for us and the best of helpers. And may Allah bless Muhammad and the progeny of Muhammad.”
This letter proves that the personality of Ja’far who claimed Imamate was questionable and in fact, the Imam considered him to be bereft of all the qualities necessary for the high position of Imamate. So much so that he has introduced Ja’far to be absolutely ignorant and foolish. Not only was he incapable to interpret the law of Shariah and other religious matters, he could not even discriminate between his right and left hand. Therefore, how could he claim the lofty status of Wilayata 
 Surah Ahqaf 46:1-6
source : alhassanain