Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1004
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.