Musnad Abu Ya'la al-Mawsili 217:13

Abu Khaythama narrated to us, Waki' narrated to us, Ja'far ibn Burqan narrated to us, from Yazid ibn al-Asamm, from Maymuna, who said: When the Messenger of Allah (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 from behind him.