Musnad Ahmad 1153:1842

Hajjaj narrated to us, he said: Shareek informed us, from Qays ibn Wahb, from an elder of Banu Sawa’ah, who said: I asked Aisha and said: "When the Messenger of Allah (peace and blessings of Allah be upon him) was in a state of major ritual impurity (janabah), would he wash his head with a washing that sufficed for that, or would he pour water over his head?" She said: "Rather, he would pour water over his head."