Maliks Muwatta Book 031, Hadith Number 093.

Section : What Is Not Permitted of Free Loans.

And Malik related to me that he had heard that a man came to Abdullah ibn Umar and said, "Abu Abd ar-Rahman, I gave a man a loan and stipulated that he give me better than what I lent him." Abdullah ibn Umar said, "That is usury." Abdullah said, "Loans are of three types| A free loan which you lend by which you desire the pleasure of Allah, and so you have the pleasure of Allah. A free loan which you lend by which you desire the pleasure of your companion, so you have the pleasure of your companion, and a free loan which you lend by which you take what is impure by what is pure, and that is usury." He said, "What do you order me to do, Abu Abd ar-Rahman?" He said, "I think that you should tear up the agreement. If he gives you the like of what you lent him, accept it. If he gives you less than what you lent him, take it and you will be rewarded. If he gives you better than what you lent him, of his own good will, that is his gratitude to you and you have the wage of the period you gave him the loan."