Musnad Ahmad 32:683

Abu Kamil narrated to us, Hammad narrated to us, Thabit narrated to us, from Anas, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) led us in a voluntary prayer." He said: "So Umm Sulaym and Umm Haram stood behind us." Thabit said: "I do not know it except that he said: 'And he placed me to his right,' and we prayed upon a mat."