It is a decided matter that the martyrdom of Imam Mohammad Taqi (a.s) took place because of poisoning and it is confirmed that he was poisoned by Motasim. Now some people have created doubts in this matter: whether this job was entrusted to Ummul Fadhl[wife of Imam(a.s) and daughter of Mamoon] or some other minister, as mentioned in some reports that the Imam was invited to a feast and then administered poison. In any case the Imam passed his life and went into the court of his creator.
However the immediate reason of his assassination was that a Qadi (Judge) awarded the sentence of cutting off the hand of a thief from his wrist as this same part is washed in ablution; but when the Imam was asked about it, he declared that it was a wrong decision. The palm of the hand is placed in Sajdah and it is for Allah. It cannot be severed; hence only the fingers should be cut off. At that time Motasim liked the Imam’s judgment, but the Qadi appealed to him that it would destroy the dignity of the Qadi and belief in his Imamate would gain strength, which is a serious risk to the regime of Motasim. This complaint enraged Motasim and he had the Imam assassinated through poison within three days.
Jilaul Uyun, Nuqush-e-Ismat pg711