Musnad Ahmad 770:77

Isma'il narrated to us, Yunus informed 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: And whenever Jarir would buy something and it was more pleasing to him than its price, he would say to its owner: "Know, by Allah, that what we have taken is more beloved to us than what we have given you," as if he intended by that to fulfill the covenant.