…he may never divorce her. (Deuteronomy 22:19)
In the previous mitzvah, we saw that if the disgruntled groom slanders his bride’s reputation vis-à-vis her virginity at the time of marriage, he has an obligation to retain her as a wife. That’s a positive mitzvah. Here we see that he also has a negative mitzvah prohibiting him from divorcing her.
The reason is what we said in Mitzvah #553: if the groom is inclined to lie in an attempt to rid himself of his bride, the prospect of it backfiring and sticking him with her forever should dissuade him.
This mitzvah only applies in Temple times because it is connected to the fine and the lashes (see previous mitzvah), so it requires a functioning Sanhedrin. It is discussed in the Talmud in tractate Sanhedrin (8a-9b) and in Kesubos (44a-46b). It is codified in the Mishneh Torah in the third chapter of Hilchos Issurei Bi’ah. This mitzvah is #359 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos.