Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 100
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 100, with English translation by Maimonides (Rambam).
להיות היולדת טמאה כנידה.
For [a woman] who gives birth to be impure [like a woman] in the niddah state.