Sahih Muslim Book 10, Hadith Number 3868.

Chapter : He who took something as loan and made its payment back, over and above that (19 approved) and best among you is one who is best in making payment.

Ma’mar b. Abdullah reported that he sent his slave with a sa’ of wheat and said to him: Sell it, and then buy with it barley. The slave went away and he got a sa’ (of barley) and a part of sa’ over and above that. When he came to Ma’mar he informed him about that, whereupon Ma’mar said to him: Why did you do that? Go back and return that, and do not accept but weight, for weight, for I used to hear from Allah’s Apostle (may peace be upon him) as saying: Wheat for wheat and like for like. He (one of the narrators) said: Our food in those days consisted of barley. It was said to him (Ma’mar) that (wheat) is not like that (barley). He replied: I am afraid these may not be similar