Reference for Megillah 39:6

ולימרו ליה אין מביאין ראיה מן הקטן חדא ועוד קאמרו ליה חדא דקטן היית ועוד אפילו גדול היית אין מביאין ראיה מן המתיר:

Why did they not say to him: Proof cannot be brought from the testimony of a minor? They gave one answer and then another. First, they said, you were a boy and moreover, even had you been an adult, proof cannot be brought from the very one who allows it.

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