Musnad Abu Ya'la al-Mawsili 217:7

Ahmad ibn Mani' narrated to us, Marwan narrated to us, from 'Abdullah ibn 'Abdullah al-'Amiri, from Yazid ibn al-Asamm, from Maymunah, the wife of the Prophet (may Allah bless him and grant him peace), she said: I heard her saying: When the Messenger of Allah (may Allah bless him and grant him peace) prostrated, he would spread his arms apart until the whiteness of his armpits could be seen from behind him. And when he sat, he would rest calmly upon his left thigh.