תּוספֿתּא אויף בראשית 5:2

A man is not permitted to marry a barren or elderly woman, an aylonit, a minor, or one who is not fit to bear children, but a woman is permitted to marry even a eunuch. Rabbi Yehuda says, one who castrates a male is liable, but [one who sterilizes] a female is exempt. Rabbi Natan said: Beit Shammai say two boys [are required to fulfill the commandment of "be fruitful and multiply"], like Moshe’s sons, as it is written: “And Moshe’s sons were Gershom and Elazar" (1 Chr. 23:15). Beit Hillel say: a son and a daughter, as it is written: “Male and female God created them” (Genesis 5:2). Rabbi Natan says: Beit Shammai say: a son and a daughter, and Beit Hillel say: a son or a daughter.