Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 76
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 76, with English translation by Maimonides (Rambam).
להקריב היולדת קרבן אחר שתטהר, שנאמר: "ובמלאות ימי טוהרה" (ויקרא יב,ו).
For a woman who gave birth to offer a sacrifice after she becomes purified, as [Leviticus 12:6] states: "And after the conclusion of her pure days...."