Qitzur Shulchan Arukh – 181:13
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.
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