Musnad Abu Ya'la al-Mawsili 225:5

Abu Khaythama narrated to us, Ishaq bin Sulayman al-Razi narrated to us, he said: I heard Ibn Abi Dhi'b, from Salih, the freed slave of al-Taw'ama, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said to his wives: "This is the Hajj, then the backs of the mats." So they all used to perform Hajj, except for Zaynab and Sawdah. They both said: "No beast shall move us after the saying of the Messenger of Allah (peace and blessings of Allah be upon him)," meaning: "This, then the backs of the mats."