Musnad Ahmad 5:377

Hushaym narrated to us, Yunus informed us, from al-Hasan, from Ali, [who said]: I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "The pen is lifted from three: from the child until he reaches puberty, from the sleeper until he wakes up, and from the afflicted [mentally ill] until he is cured."