This hadeeth comes under the following chapter: MODERATION IN PROSTRATION, PLACING THE PALMS ON THE EARTH (GROUND) AND KEEPING AWAY ELBOWS FROM THE SIDES AND THE BELLY FROM THE THIGHS WHILE PROSTRATING
Maimuna daughter of Harith reported: When the Messenger of Allaah (sallAllaahu alayhi wa sallam) 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.