Musnad Ahmad 1153:229

Yahya narrated to us, Hisham narrated to us, my father narrated to me, from Aisha, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to observe I'tikaf in the mosque, and he would incline his head toward me while I was in my chamber, and I would comb his hair, while I was menstruating."