Mishnayos Yevamos Perek 6 Mishnah 4

A High Priest may not marry a widow, whether she is a widow from betrothal or a widow from marriage. And he may not marry a grown woman. He may marry only a minor or a young woman. Rabbi Elazar and Rabbi Shimon declare a grown woman fit to marry a High Priest. And he may not marry a woman whose hymen was torn accidentally. If a priest betrothed a widow and was subsequently appointed to be High Priest, he may marry her. And there was an incident with Yehoshua ben Gamla, who betrothed Marta bat Baitos, a widow, and the king subsequently appointed him to be High Priest, and he nevertheless married her. Conversely, in the case of a widow waiting for her yavam who happened before a common priest, i.e., the priest was her yavam, and he was subsequently appointed to be High Priest, then even if he had already performed levirate betrothal with her, he may not marry her, because she is a widow. A High Priest whose brother died without children performs ḥalitza and he does not perform levirate marriage, as he may not marry a widow.

כֹּהֵן גָּדוֹל לֹא יִשָּׂא אַלְמָנָה, בֵּין אַלְמָנָה מִן הָאֵרוּסִין, בֵּין אַלְמָנָה מִן הַנִּשּׂוּאִין, וְלֹא יִשָּׂא אֶת הַבּוֹגֶרֶת. רַבִּי אֱלִיעֶזֶר וְרַבִּי שִׁמְעוֹן מַכְשִׁירִין בְּבוֹגֶרֶת. לֹא יִשָּׂא אֶת מֻכַּת עֵץ. אֵרֵס אֶת הָאַלְמָנָה, וְנִתְמַנָּה לִהְיוֹת כֹּהֵן גָּדוֹל, יִכְנֹס. וּמַעֲשֶׂה בִיהוֹשֻׁעַ בֶּן גַּמְלָא שֶׁקִּדֵּשׁ אֶת מָרְתָא בַת בַּיְתוֹס, וּמִנָּהוּ הַמֶּלֶךְ לִהְיוֹת כֹּהֵן גָּדוֹל, וּכְנָסָהּ. שׁוֹמֶרֶת יָבָם שֶׁנָּפְלָה לִפְנֵי כֹהֵן הֶדְיוֹט, וְנִתְמַנָּה לִהְיוֹת כֹּהֵן גָּדוֹל, אַף עַל פִּי שֶׁעָשָׂה בָהּ מַאֲמָר, הֲרֵי זֶה לֹא יִכְנֹס. כֹּהֵן גָּדוֹל שֶׁמֵּת אָחִיו, חוֹלֵץ וְלֹא מְיַבֵּם: