Talmud for Yevamot 15:10

נִתַּן לִי יָבָם בִּמְדִינַת הַיָּם, אָמְרָה מֵת בַּעְלִי וְאַחַר כָּךְ מֵת יְבָמִי, יְבָמִי וְאַחַר כָּךְ בַּעְלִי, נֶאֱמֶנֶת. הָלְכָה הִיא וּבַעְלָהּ וִיבָמָהּ לִמְדִינַת הַיָּם, אָמְרָה מֵת בַּעְלִי וְאַחַר כָּךְ מֵת יְבָמִי, יְבָמִי וְאַחַר כָּךְ בַּעְלִי, אֵינָהּ נֶאֱמֶנֶת. שֶׁאֵין הָאִשָּׁה נֶאֱמֶנֶת לוֹמַר מֵת יְבָמִי, שֶׁתִּנָּשֵׂא. וְלֹא, מֵתָה אֲחוֹתִי, שֶׁתִּכָּנֵס לְבֵיתוֹ. וְאֵין הָאִישׁ נֶאֱמָן לוֹמַר מֵת אָחִי, שֶׁיְּיַבֵּם אִשְׁתּוֹ. וְלֹא, מֵתָה אִשְׁתִּי, שֶׁיִּשָּׂא אֲחוֹתָהּ:

(If she said:) A yavam was given to me abroad [i.e., her mother-in-law had had a son abroad (and she had left in the status of not having a yavam)] — if she said (either): My husband died and then my yavam died, or: My yavam died and then my husband died, [permitting herself "to the marketplace" in both instances], she is believed [and is permitted "to the marketplace" as per her original status, "the mouth that forbids (by apprising us of the yavam) being the mouth that permits."] If she, her husband, and her yavam had gone abroad — if she said [either:] My husband died and then my yavam died, or: My yavam died and then my husband died, she is not believed, a woman not being believed to say: My yavam died, (for it is possible that she says so only) that she might remarry; and (she is not believed to say): My sister died, (for it is possible that she says so only) that she might enter his house (i.e., "the house" of the one who had married his sister.) And a man is not believed to say: My brother died, that he might take his wife in yibum; and (he is not believed to say): My wife died, that he might marry her sister.

Jerusalem Talmud Bava Metzia