Musnad Ahmad 1163:11

Bakr ibn Isa al-Rasibi narrated to us, Abu 'Awanah narrated to us, he said: Sulayman al-Shaybani narrated to us, he said: 'Abdullah ibn Shaddad ibn al-Had narrated to us, he said: I heard my maternal aunt, Maymunah bint al-Harith, the wife of the Prophet, may Allah bless him and grant him peace, that she "used to be menstruating while she was lying down beside the prayer place (masjid) of the Messenger of Allah, may Allah bless him and grant him peace, while he was praying on his prayer mat (khumrah). When he prostrated, the edge of his garment would touch me."