Musnad Abu Ya'la al-Mawsili 128:20

Ahmad ibn Hatim al-Tawil narrated to us, Abd al-Aziz narrated to us, Amr ibn Abi Amr informed me, from Anas: That the Prophet, peace and blessings of Allah be upon him, chose Safiyyah for himself, or by himself. When he reached al-Sahba’, he consummated the marriage with her. He commanded me, so I invited those who were around him. Sawiq (barley meal) and dates were brought, and that was the wedding feast of the Messenger of Allah, peace and blessings of Allah be upon him. He said: And I saw the Messenger of Allah, peace and blessings of Allah be upon him, gathering [a garment] for her, or shielding her, then he placed his leg for her so that she could mount.