Musnad Abu Ya'la al-Mawsili 42:335

Zuhayr narrated to us, Muhammad ibn Ubayd narrated to us, from al-A’mash, from Abu Sufyan, from Jabir, from Abu Sa’id, who said: I entered upon the Messenger of Allah (may Allah bless him and grant him peace) while he was praying upon a mat and prostrating upon it.