What are the ways to establish the Islam of a witness?

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The Islam of a witness must be established through one of four means. First, the witness must declare himself a Muslim or utter the declaration of Islam (Shahada: La ilaha illa Allah, Muhammadun Rasul Allah), as this act itself would convert him if he were not Muslim. Second, the defendant (al-mashhud 'alayhi) must confess to the witness's Islam, as this is a right against the defendant. Third, the judge's personal knowledge (khibrat al-hakim) is sufficient, analogous to its sufficiency in establishing justice (adalah). Fourth, testimony (bayyinah) confirming his Islam is acceptable.