Musnad Ahmad 32:2081

'Affan narrated to us, Shu'bah narrated to us, Ja'far ibn Ma'bad informed me, he said: I went to Anas ibn Malik, I and Humayd ibn 'Abd al-Rahman. He said: So I heard Anas say: When we used to pledge allegiance to the Messenger of Allah, may Allah bless him and grant him peace, he would instruct us: "In what you are able."