From Sahih Bukhari (Hadith 4551):
Narrated `Abdullah bin Abu `Aufa:A man displayed some merchandise in the market and took an oath that he had been offered a certain price for it while in fact he had not, in order to cheat a man from the Muslims. So then was revealed:-- "Verily! Those who purchase a small gain at the cost of Allah's Covenant and their oaths
Reference: Bukhari 4551