Musnad Abu Ya'la al-Mawsili 148:77

Abu Bakr ibn Abi Shaybah narrated to us, Waki' narrated to us, from Shu'bah, from 'Attab, the freed slave of Hurmuz, who said: I heard Anas ibn Malik saying: We pledged allegiance to the Messenger of Allah (peace and blessings of Allah be upon him) to hear and obey. He said: "In what you are able."