Musnad Ahmad 33:904

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, Bashir ibn Abi Bashir, the freed slave of the family of al-Zubayr, narrated to me, he said: I heard al-Hasan ibn Muhammad ibn Ali ibn Abi Talib asking Jabir ibn Abdullah al-Ansari, the brother of Banu Salama, about the ritual bath (ghusl) for major ritual impurity (janaba). Jabir said: "The Messenger of Allah, peace and blessings of Allah be upon him, used to scoop water over his head three scoops with his hands, then he would pour water over his skin." Al-Hasan said to him: "Indeed, the hair of my head is thick, and I fear that three scoops with my hands will not wash it." Jabir said to him: "The head of the Messenger of Allah, peace and blessings of Allah be upon him, was thicker and better than your head."