Sahih Muslim Book 4, Hadith Number 1004.

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

Maimuna daughter of Harith reported: When the Messenger of Allah (may peace be upon him) prostrated, he kept his hands so much apart from each other that when it was seen from behind the armpits became visible. Waki’ said: That is their whiteness.