Musnad Abu Ya'la al-Mawsili 109:312

Narrated to us Ibn Numayr, narrated to us Muhammad ibn Ubayd and Ya'la, from al-A'mash, from Abu Sufyan, from Jabir, from Abu Sa'id, who said: I entered upon the Prophet, peace and blessings of Allah be upon him, while he was praying upon a mat and prostrating upon it. And I entered upon him while he was praying wrapped in a garment.