Musannaf Ibn Abi Shaybah

Hazrat Haroon narrates that I asked Hazrat Hakam and Hazrat Hammad: A person gave his wealth to another as Mudarabah, and established witnesses over it. Later, when he came to collect his wealth from him, the other said, "I have already returned the wealth." Hakam says that he will have to establish witnesses that he has returned the wealth, just as the owner of the wealth had established witnesses over him. And Imam Muhammad says that just as his word is accepted in other matters, in this matter too his word will be accepted (i.e., witnesses will not be demanded).