Reference for Arakhin 34:34

דבר המכפר מאי ניהו חטאת

And if you will say: Indeed so! Surely it was taught: If a man knew testimony [to give] for another before he became his son-in-law, and then became his son-in-law; or if he then could hear and now became deaf; could see and now became blind; was of sound mind then and now became stupid, then he is disqualified [as witness].

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