Positive commandment · Mitzvat Aseh #212 of 613
To be fruitful and multiply
לפרות ולרבות
The verse · Bereshit 9:7
וְאַתֶּם פְּרוּ וּרְבוּ שִׁרְצוּ בָאָרֶץ וּרְבוּ־בָהּ
“And as for you, be fruitful, and multiply; bring forth abundantly in the earth, and multiply in it.”
The Rambam’s words
To be fruitful and multiply, as [Genesis 9:7] states: "Be fruitful and multiply."
לפרות ולרבות, שנאמר: "פרו ורבו" (בראשית ט, ז).
Why · Sefer HaChinuch #1
Procreation
“It is from the roots of this commandment (i.e. the reason behind this commandment) that the world should be settled (Gittin 41b) because G-d wants the world to be settled, as it says (Isaiah 45:18), “I did not create it for naught, but [rather] formed it for habitation.””
Who & when
“And [it] is practiced in every place and at all times (Kiddushin 36b), and a person is obligated to be involved with it from when it is fitting for him, and this is the age which the Sages (Avot 5:21) designated to marry a woman.”
Where in the Mishneh Torah
Hilchot Marriage הלכות אישות