Praise be to Allaah.
The mutawaatir hadeeth is that which was narrated by a large number in such a way that they could not have agreed on a lie, from a similarly large number, and so on until the end of the isnaad (chain of narrators), and their report is based on one of the five senses. Hence there is no room for confusion or error on the part of the narrator, because if we assume that a narrator had a poor or weak memory, the large number of narrators and the fact that they all narrated the same report strengthens the hadeeth..
As for the narrator of the hadeeth himself, if he suffered from mental problems, the scholars of hadeeth stipulated stringent conditions for accepting the report of a narrator so as to confirm that he was free of any flaws or mistakes. They stipulated that he should be a Muslim of sound mind with a precise memory if he narrated from memory, or that he should be known to be careful and precise in his writing if he narrated from his writing, and that he should be a man of piety who kept away from sin and was not known for evildoing or openly committing sin.
They also stipulated that he should be free of anything that would undermine dignity; his conduct and attitude should be proper and he should not behave in an odd or suspicious manner, such as wearing improper clothing in a public gathering, and the like.
Once they established that the narrator was free of these faults, they would compare his reports to the reports of other scholars who were known for precision in narration, so as to make sure that his hadeeth was free from any oddities or flaws, i.e., errors or anything that contradicted the reports of anyone who was more credible than him, either in the number of reports that he narrated or in his memory or precision. This matter could be so subtle that it is very difficult for many seekers of knowledge to understand, but they should try their utmost to research and examine the matter until they become certain as to the soundness of the narrator or the soundness of his reports. Hence we find that the scholars stated that the conditions of a hadeeth being saheeh (sound) are five:
1. Good character of all the narrators, which means that all of them should be free of anything that may be regarded as evildoing and anything that may undermine their dignity.
2. Precise memory of the narrators who narrated the hadeeth.
3. No interruptions in the chain of narrators (isnaad) from beginning to end, in the sense that each narrator heard it from the narrator above him in the chain.
4. The hadeeth should be free of anything odd in its isnaad or text. What is meant by odd is anything in which the narrator contradicts a narrator who is more credible than him.
5. The hadeeth should be free of flaws in its isnaad or text. The flaw is a subtle reason that could undermine the soundness of the hadeeth. This is only known to well-versed scholars.
Thus it becomes clear that there is no room for accepting the hadeeth of the narrator who suffered from any mental disorder that could affect his memory and narration. The same applies if he has any psychological problems that means that he is not qualified to narrate a hadeeth or teach it; in that case he is not to be relied upon when he narrates from the Messenger of Allah (blessings and peace of Allah be upon him). Rather the reports of any narrator cannot be accepted unless the scholars of this field testify that he was well versed and qualified, and that his hadeeth is acceptable. And Allah knows best.
See: Tadreeb al-Raawi by al-Suyooti, 1/68-75, 155; al-Nukat ala Ibn al-Salaah by Ibn Hajar, 1/480.