Musnad Ahmad 5:759

'Affan narrated to us, Hammad narrated to us, from 'Ata' ibn al-Sa'ib, from Abu Zabyan al-Janbi, that 'Umar ibn al-Khattab, may Allah be pleased with him, was brought a woman who had committed adultery, so he ordered her to be stoned. They took her to stone her, and 'Ali, may Allah be pleased with him, met them and said: "What is this?" They said: "She committed adultery, and 'Umar ordered her to be stoned." 'Ali snatched her from their hands and turned them back. They returned to 'Umar, may Allah be pleased with him, who said: "What turned you back?" They said: "'Ali, may Allah be pleased with him, turned us back." He said: "'Ali did not do this except for something he knew." He sent for 'Ali, and he came appearing as if he were angry. He said: "What is the matter with you that you turned these people back?" He said: "Did you not hear the Messenger of Allah, peace and blessings of Allah be upon him, say: 'The pen is lifted from three: from the sleeper until he wakes up, from the child until he grows up, and from the afflicted person until he regains his reason'?" He said: "Yes." 'Ali, may Allah be pleased with him, said: "For this woman is afflicted with the affliction of the tribe of so-and-so, so perhaps he came to her while she was in that state." 'Umar said: "I do not know." He said: "And I do not know." So he did not stone her.