And if you do not do it, then take a notice of war from Allâh and His Messenger but if you repent, you shall have your capital sums. Deal not unjustly (by asking more than your capital sums), and you shall not be dealt with unjustly (by receiving less than your capital sums).
Narrated by: Ibn 'Umar
A man was often cheated in buying. The Prophet said to him, "When you buy something, say (to the seller), No cheating." The man used to say so thenceforward .