Musnad Ahmad 1163:13

Muhammad ibn Fudayl narrated to us, al-Shaybani narrated to us, from Yazid ibn al-Asamm, from Maymunah, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to pray on a khumrah (a small prayer mat made of palm leaves), and he would prostrate, and his garment would touch me while I was beside him, and I was menstruating."