Bava Batra 134B

ליורשו ⁠– פשיטא! לפטור את אשתו מן היבום אצטריכא ליה.

The Talmud asks: Isn’t it obvious that his claim is deemed credible with regard to someone inheriting from him? Since he could have given this person his property as a gift, it need not be stated that his claim is accepted with regard to his inheritance. The Talmud responds: It was necessary for the Mishnah to state that his claim is deemed credible with regard to rendering his wife exempt from levirate marriage despite the fact that it is not in his power to render her exempt, but the halakha of inheritance is not a novelty.

הא נמי תנינא ⁠– מי שאמר בשעת מיתתו: ״יש לי בנים״⁠ – נאמן. ״יש לי אחים״⁠ – אינו נאמן!

The Talmud asks: We already learned this in a Mishnah (Kiddushin 64a) as well: One who said at the time of his death: I have children, is deemed credible, and his wife is thereby exempt from levirate marriage. If he said: I have brothers, and his wife therefore must enter levirate marriage, he is not deemed credible.

התם ⁠– דלא מוחזק לן באח, הכא ⁠– אף על גב דמוחזק ליה באח.

The Talmud answers: There, in that Mishnah, it is a case where he is not presumed by us to have a brother. Therefore, his wife is already presumed to be exempt from levirate marriage, and his claim that he has a son merely substantiates this presumption. Here, the Mishnah adds a novelty that even if he is presumed to have a brother, his claim that he has a son is accepted, and his wife is thereby exempted from levirate marriage.

אמר רב יוסף אמר רב יהודה אמר שמואל, מפני מה אמרו: ״זה בני״⁠ – נאמן? הואיל ובעל שאמר: ״גרשתי את אשתי״⁠ – נאמן.

Rav Yosef says that Rav Yehuda says that Shmuel says: For what reason did the rabbis say that one who says: This is my son, is deemed credible? Since a husband who says: I divorced my wife, is deemed credible, and his wife is thereby exempt from levirate marriage, he is deemed credible with regard to this claim as well.