Related for Kiddushin 4:8
הָאוֹמֵר, בְּנִי זֶה מַמְזֵר, אֵינוֹ נֶאֱמָן. וַאֲפִלּוּ שְׁנֵיהֶם אוֹמְרִים עַל הָעֻבָּר שֶׁבְּמֵעֶיהָ מַמְזֵר הוּא, אֵינָם נֶאֱמָנִים. רַבִּי יְהוּדָה אוֹמֵר, נֶאֱמָנִים:
If one says: "This son of mine is a mamzer," he is not believed. [For he is kin to him, and thus not believed to testify.] And even if both of them [husband and wife] say about a fetus in her womb that it is a mamzer, they are not believed. [Not only when the father alone testifies that he is a mamzer is he not believed, not being certain, but even when his mother, who is certain, testifies, she is not believed, even not concerning a fetus in her womb, who was never in a status of kashruth.] R. Yehudah says: They are believed. [The rationale of R. Yehudah: It is written (Deuteronomy 21:17): "But the first-born … shall he (the father) recognize" — he shall cause him to be recognized by others, whence it is derived that the father is believed to render his son pasul (unfit), and the mother is not believed to do so. And it is only concerning his son that he is believed, but not concerning the son of his son. If his son had sons, he is not permitted to render them pasul. The halachah is in accordance with R. Yehudah.]