Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 212
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 212, with English translation by Maimonides (Rambam).
לפרות ולרבות, שנאמר: "פרו ורבו" (בראשית ט, ז).
To be fruitful and multiply, as [Genesis 9:7] states: "Be fruitful and multiply."