Musnad Ahmad 1060:32

Sufyan narrated to us, from Zayd ibn Aslam, from Ibrahim ibn Abdullah ibn Hunayn, from his father, who said: Al-Miswar and Ibn Abbas differed—and he said once: they disputed—regarding the *muhrim* (one in the state of ihram) pouring water over his head. He said: So they sent [a messenger] to Abu Ayyub, "How did you see the Messenger of Allah, peace and blessings of Allah be upon him, wash his head?" He said: "Like this, front to back," and Sufyan demonstrated it.