Musnad Abu Ya'la al-Mawsili 251:2

'Amr ibn Muhammad al-Naqid narrated to us, Isma'il ibn 'Ulayyah narrated to us, Yunus ibn 'Ubayd narrated to us, from 'Amr ibn Sa'id, from Abu Zur'ah ibn 'Amr ibn Jarir, who said: Jarir said: "I pledged allegiance to the Messenger of Allah, may Allah bless him and grant him peace, to listen and obey, and to offer sincere advice to every Muslim." He said: So when he would buy something, it would be more pleasing to me than its price. He would say to its owner: "By Allah, what we take from you is more beloved to us than what we give you." He said: He meant by that the fulfillment of the pledge.