Musnad Ahmad 33:920

Ibrahim narrated to us, Rabah narrated to us, from Ma'mar, from Zayd ibn Aslam, from Ubayd Allah ibn Miqsam, who said: I was with al-Hasan ibn Muhammad ibn Ali, and he asked Jabir ibn Abd Allah about the ritual bath for major impurity (janabah). He said: "You moisten the hair and wash the skin." He said: "My head has a lot of hair." He said: "The Prophet, peace and blessings of Allah be upon him, used to pour three handfuls of water over his head." Al-Hasan ibn Muhammad said: "My head has a lot [of hair]." He said: "The head of the Messenger of Allah, peace and blessings of Allah be upon him, had more, and was better."