Musnad Ahmad 1157:1

Hajjaj and Yazid ibn Harun narrated to us, they said: Ibn Abi Dhi’b informed us; and Ishaq ibn Sulayman said: I heard Ibn Abi Dhi’b, from Salih, the freed slave of al-Taw’amah, from Abu Hurayrah, that the Messenger of Allah (peace and blessings of Allah be upon him) said to his wives in the year of the Farewell Pilgrimage: "This [is the pilgrimage], then the backs of the mats." He said: So they all used to perform Hajj, except Zaynab bint Jahsh and Sawdah bint Zam’ah. They used to say: "By Allah, no beast shall move us after we have heard that from the Messenger of Allah (peace and blessings of Allah be upon him)." Ishaq ibn Sulayman said in his narration: They both said: "By Allah, no beast shall move us after the saying of the Messenger of Allah (peace and blessings of Allah be upon him): 'This, then the backs of the mats.'" And Yazid said: "After we heard that from the Messenger of Allah (peace and blessings of Allah be upon him)."