Musannaf Ibn Abi Shayba 13:2805

Abu Bakr narrated to us, he said: Ibn Abi Za'idah narrated to us, from Zakariyya, he said: I said to 'Amir: "A man borrows a loan, and when he pays it back, he gives him a gift better than it." He said: "There is no harm in it, as long as he does not stipulate it, or give it seeking that [in return]."