The 21st prohibition is that the one whom to meisis tried to incite is forbidden from withholding anything he knows which would cause the meisis to be punished.

The source of this commandment is G‑d's statement (exalted be He),1 "And do not cover up for him."

The explanation of this is brought, "The verse,2 'And do not try to cover up,' means that if you know of something against him, you may not remain silent."