Musnad Ahmad 32:263

Waki' narrated to us, Shu'bah narrated to us, from 'Attab, the freed slave of Ibn Hurmuz, who said: I heard Anas bin Malik say: We pledged allegiance to the Messenger of Allah, may Allah bless him and grant him peace, to listen and obey. He said: "In what you are able to do."