The 23rd prohibition is that we are forbidden from rebuilding an ir ha'nidachas.1

The source of this commandment is G‑d's statement (exalted be He),2 "[The city] shall then remain an eternal ruin; do not ever have it rebuilt."

Anyone who builds any part of it, i.e. [helps] make it into the same city it was before,3 is punished by lashes.

The details of this mitzvah have been explained in the tenth chapter of Sanhedrin.