Musnad Ahmad 5:617

Muhammad ibn Ja'far narrated to us, Sa'id narrated to us, from Qatadah, from al-Hasan: That 'Umar ibn al-Khattab intended to stone a woman who was insane. 'Ali said to him, "You have no right to do that." He said, "I heard 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 reaches puberty, and from the insane person until he recovers—or regains his reason.'" So 'Umar refrained from [punishing] her.