Reference for Arakhin 35:5

(ויקרא ה, א) או ראה אם לא יגיד בראייה והגדה תלא רחמנא מילתא והא איכא

But then what is the need of: 'He being a witness'? - Because of what has been taught: If he saw a company of men standing, among whom are his witnesses, and he says: I adjure you that if you know a testimony on my behalf you come and testify for me, one might have assumed that they then are obliged [to do so], therefore the text states: 'He being a witness', whilst he has not singled out his witnesses.

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