Musnad Ahmad 25:1465

Waki’ narrated to us, from Muhammad ibn Qays, from al-Hakam, from Sa’id ibn Jubayr, from Ibn Abbas, who said: I spent the night at the house of my maternal aunt, Maymunah. He said: The Prophet, peace and blessings of Allah be upon him, stood up during the night and performed ablution. He said: So I stood up and performed ablution, then he stood and prayed, and I stood behind him, or to his left, so he turned me around until he placed me to his right.