Sahih Muslim Book 10, Hadith Number 3896.

Abu Rafi’ reported that Allah’s Messenger (may peace be upon him) took from a man as a loan a young camel (below six years). Then the camels of Sadaqa were brought to him. He ordered Abu Rafi’ to return to that person the young camel (as a return of the loan). Abu Rafi’ returned to him and said: I did not find among them but better camels above the age of six. He (the Holy Prophet) said: Give that to him for the best men are those who are best in paying off the debt.