Musnad Ahmad 304:1

Yahya ibn Sa'id narrated to us, from Hisham—meaning Ibn 'Urwah—who said: My father narrated to me, from 'Umar ibn Abi Salamah. And Waki' said: Hisham narrated to us, from his father, from 'Umar ibn Abi Salamah, who said: "I saw the Messenger of Allah, peace and blessings of Allah be upon him, praying in a single garment." Waki' said: "In the house of Umm Salamah, in a garment, having cast its two ends over his shoulders, in the house of Umm Salamah."