יג: הָא דְּעֵד אֶחָד מֵעִיד, זֶהוּ דַוְקָא בְּדָבָר שֶׁבְּמָמוֹן, דְּמַהַנֵי גַם עֵד אֶחָד לְעִנְיַן שְׁבוּעָה. וְכֵן בִּדְבַר אִסוּר, אִם עֲדַיִן לֹא נַעֲשָׂה הָאִסּוּר, יָעִיד כְּדֵי לְאַפְרוּשֵׁי מֵאִסּוּרָא. אֲבָל אִם כְּבָר נַעֲשֶׂה הָאִסּוּר, לֹא יָעִיד עֵד אֶחָד. דְּכֵיוָן דְּעֵד אֶחָד אֵינוֹ נֶאֱמָן, אֵינוֹ אֶלָּא כְּמוֹצִיא ֹשֵם רָע עַל חֲבֵרוֹ
That where one witness testifies, that is specifically in a financial matter, since one witness is also of help with respect to requiring the defendant to take an oath [that he doesn't owe the money, whereas two would require him to pay]. Similarly in a matter of a prohibition, it the prohibition wasn’t done yet, he should testify in order to separate people from [possible] violation. But if the prohibition was already done, a single witness should not testify, since a single witness is not believed, it is as though he did nothing but slander his friend.
