Musnad Ahmad 1153:399

Husayn narrated to us, Sharik narrated to us, from Qays ibn Wahb, from an elder of Banu Sawa’ah, who said: I asked ‘Aishah, and I 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 [agent] and suffice with that, or would he pour water over his head?" She said: "Rather, he would pour water over his head."