Hilchos Rechilus, chapter 1, section 7

Rechilus is certainly forbidden if keeping silent will only lead to insults and no financial loss. A person must remind himself that this will lead in the next world to him being considered among those who love G-d and his face will shine like the sun as the sages say (Yuma 23): “Those who are shamed and do not shame, hear their disgrace and do not reply… of them the verse says (Shoftim 5:31), And those who love Him are like the sun going out in its might. This verse applies even more if one is insulted due to keeping mitzvos. See what we spoke of at length earlier in Laws of Lashon Hara (1:6).