Reference for Yevamot 13:12
יָבָם קָטָן שֶׁבָּא עַל יְבָמָה קְטַנָּה, יִגְדְּלוּ זֶה עִם זֶה. בָּא עַל יְבָמָה גְדוֹלָה, תְּגַדְּלֶנּוּ. הַיְבָמָה שֶׁאָמְרָה בְּתוֹךְ שְׁלֹשִׁים יוֹם, לֹא נִבְעָלְתִּי, כּוֹפִין אוֹתוֹ שֶׁיַּחֲלֹץ לָהּ. לְאַחַר שְׁלֹשִׁים יוֹם, מְבַקְשִׁים הֵימֶנּוּ שְׁיַּחֲלֹץ לָהּ. וּבִזְמַן שֶׁהוּא מוֹדֶה, אֲפִלּוּ לְאַחַר שְׁנֵים עָשָׂר חֹדֶשׁ כּוֹפִין אוֹתוֹ שֶׁיַּחֲלֹץ לָהּ:
If one vows, in her husband's lifetime, to derive no enjoyment from her yavam, he is compelled to give her chalitzah. [For (when she made the vow) it did not enter her mind that her husband would die and that she would fall before him for yibum, and she receives her kethubah.] After her husband's death, he is requested to give her chalitzah. And if she intended this (to shun yibum), even in her husband's lifetime, he is requested to give her chalitzah. [After her husband's death, she is a rebel (in vowing thus), and we rule that a writ of rebellion is written against one awaiting yibum (and refusing it). We request that he give her chalitzah, and he gives her her kethubah if he wishes to. And if he does not wish to give her chalitzah and she rebels against him and consents to lose her kethubah, we compel him to give her chalitzah.]