Musannaf Ibn Abi Shayba 37:251

Abu Usama narrated to us, from Ibn 'Awn, from Umayr ibn Ishaq, who said: I and a man entered upon al-Hasan ibn Ali to visit him. He began saying to that man, "Ask me before you do not ask me." The man said, "I do not want to ask you anything; may Allah grant you health." He said: So he stood up and entered the privy, then he came out to us and said, "I did not come out to you until I spat out a portion of my liver, which I was turning over with this stick. I have been given poison to drink many times, but there is nothing more severe than this time." He said: We went to him the next day, and he was in the market. He said: Al-Husayn came and sat at his head and said, "O my brother, who is your companion [who poisoned you]?" He said, "Do you intend to kill him?" He said, "Yes." He said, "If it is the one I think, then Allah’s vengeance is more severe. And if he is innocent, then I do not like for an innocent person to be killed."