Reference for Gittin 57:4

ת"ש שמע מב"ד של ישראל שהיו אומרים איש פלוני מת איש פלוני נהרג ישיאו את אשתו מקומנטריסים של עובדי כוכבים איש פלוני מת איש פלוני נהרג אל ישיאו את אשתו

and as for your question why in such a case [if the report comes] from a heathen court is she not allowed to marry again, seeing that it is a recognised principle with us that the word of a heathen speaking without ulterior motive is to be accepted, [the answer is that] this applies only to a matter in which the heathen has not participated, but where the matter is one in which they have themselves participated, he is prone to indulge in falsehood.

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