Sahih Muslim Book 4, Hadith Number 1003.

Chapter : How the limbs should work in prostration and forbiddance to fold clothing and hair and plaiting of hair in the prayer.

Maimuna, the wife of the Apostle of Allah (may peace be upon him), reported: When the Messenger of Allah (may peace be upon him) prostrated himself, he spread his arms, i.e. he separated them so much that the whiteness of his armpits became visible from behind and when he sat (for Jalsa) he rested on his left thigh.