Musnad Ahmad 1163:54

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Sulayman al-Shaybani, from 'Abdullah ibn Shaddad, from his maternal aunt Maymunah, that the Prophet (peace and blessings of Allah be upon him) "used to pray on the khumrah (a small prayer mat made of palm leaves)."