Musnad Ahmad 1163:23

Waki' narrated to us, Ja'far ibn Burqan narrated to us, from Yazid ibn al-Asamm, from Maymunah, who said: "When the Prophet, peace and blessings of Allah be upon him, prostrated, he would keep his arms away from his sides until the whiteness of his armpits could be seen by those behind him."