Musnad Ahmad 27:833

'Abd al-Rahman narrated to us, Sufyan and Shu'bah narrated to us, from 'Abdullah ibn Dinar, from Ibn 'Umar, who said: When we would pledge allegiance to the Prophet, may Allah bless him and grant him peace, to listen and obey, he would instruct us or dictate to us: "As much as you are able."