Musnad Ahmad 1163:36

Kathir ibn Hisham narrated to us, he said: Ja'far and Ali ibn Thabit narrated to us, he said: Ja'far ibn Burqan narrated to me, he said: Yazid—meaning Ibn al-Asamm—narrated to us, from Maymunah, the wife of the Prophet, may Allah bless him and grant him peace, she said: "When the Messenger of Allah, may Allah bless him and grant him peace, would prostrate, he would keep his arms away from his sides until the whiteness of his armpits could be seen from behind him."