Reference for Ketubot 200:6

אי מההיא הוה אמינא ה"מ שליח אבל ב"ד לא קמ"ל

If [the ruling were to be derived] from that [Mishnah alone] I might have thought that it applied only to an agent but not to a court; thus he teaches us [that the same law applies to the court].

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