Musnad Ahmad 32:1173

Yazid narrated to us, Shu'bah informed us, he said: I heard 'Attab, the freed slave of Ibn Hurmuz, saying: I accompanied Anas ibn Malik on a ship, and I heard him saying: "I pledged allegiance to the Messenger of Allah, may Allah bless him and grant him peace, with this hand of mine—and he gestured with his right hand—to listen and to obey, as much as I am able."