I'm looking for the source of the mitzvah of going to the mikvah after being in niddah. I've been directed by others to parts in vayikra that speak about niddah, but there it says a woman should bring a korban when she is finished being in niddah. What I'm looking for is where the mitzvah of going to the mikvah comes from. Thank you for your help.
The requirement for a niddah to immerse is a Torah level, D'oraita, law. However, it is not explicitly stated in any verse in Torah. This is an instance where the Rabbinic tradition is essential for a proper understanding of the Torah text.
On another note, a zavah brings a sacrifice when she exits that status (VaYikra 15:25-30), but a niddah does not (VaYikra 15:19-24). Again, the Rabbinic tradition is important in understanding the verses.