Musnad Ahmad 947:10

Sa'id ibn Mansur narrated to us, 'Abd al-'Aziz ibn Muhammad narrated to us, from Zayd ibn Aslam, from Waqid ibn Abi Waqid al-Laythi, from his father: That the Prophet, peace and blessings of Allah be upon him, said to his wives during his Hajj: "This, then the backs of the mats."