Abu Dawud Book 36, Hadith Number 4927.

Narated By Jarir : I swore allegiance to the Apostle of Allah (PBUH) promising to hear and obey, and behave sincerely towards every Muslim. AbuZur’ah said: Whenever he sold and bought anything, he would say: What we took from you is dearer to us than what we gave you. So choose (as you like).