Mishnayos Niddah Perek 5 Mishnah 4

A girl who is three years and one day old, whose father arranged her betrothal, is betrothed through intercourse, as the halakhic status of intercourse with her is that of intercourse in all halakhic senses. And in a case where the childless husband of a girl three years and one day old dies, if his brother the yavam engages in intercourse with her, he acquires her as his wife; and if she is married, a man other than her husband is liable for engaging in intercourse with her due to violation of the prohibition against intercourse with a married woman. And if she is impure due to menstruation, she imparts impurity to one who engages in intercourse with her who then renders impure all the layers of bedding beneath him, rendering them impure like the upper bedding covering a zav, in the sense that it assumes first-degree ritual impurity and does not become a primary source of ritual impurity, and it renders impure food and drink, but it does not render impure people and vessels. If she marries a priest, she may partake of teruma, like any other wife of a priest; if she is unmarried and one of the men who are unfit for the priesthood, e.g., a mamzer or ḥalal, engaged in intercourse with her, he disqualifies her from marrying into the priesthood, and if she is the daughter of a priest, she is disqualified from partaking of teruma. Finally, if one of all those with whom relations are forbidden, as stated in the Torah, e.g., her father or her husband’s father, engaged in intercourse with her, they are executed by the court for engaging in intercourse with her, and she is exempt, because she is a minor. If the girl is less than that age, younger than three years and one day, the status of intercourse with her is not that of intercourse in all halakhic senses; rather, it is like placing a finger into the eye. Just as in that case, the eye constricts, sheds tears, and then returns to its original state, so too, in a girl younger than three years and one day old, the hymen returns to its original state.

בַּת שָׁלשׁ שָׁנִים וְיוֹם אֶחָד, מִתְקַדֶּשֶׁת בְּבִיאָה. וְאִם בָּא עָלֶיהָ יָבָם, קְנָאָהּ. וְחַיָּבִין עָלֶיהָ מִשּׁוּם אֵשֶׁת אִישׁ, וּמְטַמְּאָה אֶת בּוֹעֲלָהּ לְטַמֵּא מִשְׁכָּב תַּחְתּוֹן כָּעֶלְיוֹן. נִשֵּׂאת לְכֹהֵן, תֹּאכַל בַּתְּרוּמָה. בָּא עָלֶיהָ אַחַד מִן הַפְּסוּלִים, פְּסָלָהּ מִן הַכְּהֻנָּה. בָּא עָלֶיהָ אַחַד מִכָּל הָעֲרָיוֹת הָאֲמוּרוֹת בַּתּוֹרָה, מוּמָתִין עַל יָדָהּ, וְהִיא פְטוּרָה. פָּחוֹת מִכָּן, כְּנוֹתֵן אֶצְבַּע בָּעָיִן: