Sahih Muslim Book 4, Hadith Number 1041.

Chapter : The excellence of the prayer and the way it is begun and the excellence of Ruku’ and moderation in it and prostration and moderation in it, etc.

Maimuna, the wife of the Apostle (may peace be upon him), reported: The Messenger of Allah (may peace be upon him) said prayer and I (lay) opposite to him while I was in menses. Sometimes his clothes touched me when he prostrated.