Musnad Ahmad 1153:677

'Affan narrated to us, he said: Hammad narrated to us, from Hammad, from Ibrahim, from al-Aswad, from 'Aishah, from the Prophet, peace and blessings of Allah be upon him, who said: "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 regains his reason." And Hammad said: "And from the mentally deficient until he regains his reason."