Musnad Ahmad 1060:81

Muhammad ibn Bakr narrated to us, Ibn Jurayj narrated to us, Hajjaj narrated to us, from Ibn Jurayj, and Rawh narrated to us, Ibn Jurayj narrated to us, Zayd ibn Aslam informed me, from Ibrahim ibn Abdullah ibn Hunayn, the freed slave of the family of Abbas—and Rawh said: the freed slave of Abbas—that he informed him from his father, Abdullah ibn Hunayn, who said: I was with Ibn Abbas and al-Miswar at al-Abwa’, and we talked until we mentioned the washing of the head by one in the state of ihram. Al-Miswar said: "No," and Ibn Abbas said: "Yes." So Ibn Abbas sent me to Abu Ayyub, saying: "Your nephew, Abdullah ibn Abbas, sends you greetings and asks you: How did the Messenger of Allah, peace and blessings of Allah be upon him, wash his head while in the state of ihram?" He said: "I found him bathing between the two pillars of a well, having covered himself with a cloth. When I made myself known to him, he drew the cloth to his chest until his face appeared to me, and I saw him while a person was standing and pouring water over his head." He said: "Abu Ayyub gestured with his hands over his entire head, over his whole head, and he brought them forward and backward." Al-Miswar said to Ibn Abbas: "I will never dispute with you again." Hajjaj and Rawh said: "When I identified myself to him and asked him, he drew the cloth to his chest until his head and face appeared to me, and a person was standing."