What is the religious ruling on a person recalling their life before birth, and is concealing this matter a sin if the the person is certain of it?

There is no harm in concealing the extraordinary experiences one encounters; in fact, concealing them might be better. Knowledge of what preceded birth is impossible, and the fundamental principle is that a servant has no knowledge of what occurred before their birth, based on the Almighty's saying: "And Allah brought you forth from the wombs of your mothers while you knew nothing" (An-Nahl: 78).