Tosefta su Genesi 5:2

...Rabbi Natan said: Beit Shammai say two boys [to fulfill pru urvu], like Moshe’s sons, as it is written: “And Moshe’s sons were Gershom and Elazar" (Chronicles 1 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 said: Beit Shammai say: a son and a daughter, and Beit Hillel say: a son or a daughter....