Musnad Ahmad 1153:2259

Muhammad ibn Yazid narrated to us, from Sufyan—meaning Ibn Husayn—from al-Zuhri, from Urwah, from Aisha, who said: "When the Messenger of Allah (peace and blessings of Allah be upon him) was in I'tikaf in the mosque, he would not enter the house except for a need." She said: "So I would wash his head while there was a threshold between me and him."