Yevamos15: 9
,,נִתַּן לִי בֵן בִּמְדִינַת הַיָּם”, וְאָמְרָהּ ,,מֵת בְּנִי וְאַחַר כָּךְ מֵת בַּעְלִי” — נֶאֱמֶנֶת. ,,מֵת בַּעְלִי וְאַחַר כָּךְ מֵת בְּנִי” — אֵינָהּ נֶאֱמֶנֶת, וְחוֹשְׁשִׁים לִדְבָרֶיהָ, וְחוֹלֶצֶת וְלֹא מִתְיַבֶּמֶת.
[If a woman said,] “I was granted a son overseas,” and she added, “my son died and then my husband died” —- she is believed. [If she said,] “My husband died and then my son died” —- she is not believed. However, we consider her statement, and she must [therefore] perform chalitzah, but not yibum.