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."
Muwatta Malik Book 31, Hadith 93