Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1392
Maimuna, the wife of the Apostle of Allah (may peace be upon him), reported: The Messenger of Allah (may peace be upon him) said prayer while I was by his side, and at times when he prostrated his cloth touched me, and he prayed on a small mat.