Musnad Ahmad 1153:2229

Yunus narrated to us, Hammad narrated to us, from Hammad, from Ibrahim, from al-Aswad, from Aisha, who said: "The Prophet, peace and blessings of Allah be upon him, would be in I'tikaf in the mosque, and he would extend his head out, and I would wash it with al-khatmi (marshmallow plant), while I was menstruating."