This is because one who acknowledges a debt (2) admits that it is upon him, whereas one who acknowledges a loan does not necessarily admit to that, as it is possible he borrowed from him and then repaid him. There is a fourth report from him [Ahmad] that if he hears something and is called to testify to it (2), he has the choice: if he wishes, he may testify, and if he wishes, he may not. He said: 'But it is mandatory upon him if he is called to witness to testify if he is summoned, {And let the witnesses not refuse when they are called upon} (4).' He said: 'If they are asked to bear witness.' Ibn Abi Musa said: 'If a man hears a person acknowledge a right to another, and he did not say, "Bear witness against me regarding that," it is permissible (5) for the witness (6) to testify against him by saying, "I bear witness that I was present for the acknowledgement of so-and-so regarding such-and-such." But he does not say, "I bear witness to his acknowledgement." And if he hears him saying, "I borrowed from so-and-so" or "I received from so-and-so," it is not permitted for him to testify to it. [And the correct view is the first, because the witness testifies to what he knows, and knowledge was acquired by him through his hearing, so it is permissible for him to testify to it] (7), just as it is permissible for him to testify to actions he has seen.' The Qadi mentioned that regarding actions, there are two reports: one is that he does not testify to it until the one against whom the testimony is made says, "Bear witness." If he intended this generally for all actions, it is not correct, because that would lead to preventing testimony upon them entirely; for the usurper does not say to anyone, "Bear witness against me that I am usurping," nor does the thief, the adulterer, the killer, or those like them. Abu Bakra and his companions testified against al-Mughira regarding adultery, and Umar did not ask, "Did he ask you to bear witness or not?" (8). Nor did he say that to those (9) who testified against Qudama regarding the drinking of alcohol (10), nor did Uthman (11) say that to those who testified to that (12) against al-Walid ibn Uqba (13).