Musnad Ahmad 947:15

Muhammad ibn al-Nawshajan—who is Abu Ja'far al-Suwaydi—narrated to us, al-Darawardi narrated to us, Zayd ibn Aslam narrated to me, from Ibn Abi Waqid al-Laythi, from his father: That the Prophet, peace and blessings of Allah be upon him, said to his wives during the Farewell Pilgrimage: "This, then the backs of the mats."