The 20th prohibition is that the one whom the meisis tried to incite is forbidden from trying to acquit the meisis. Even if he knows of an argument in his favor, he is forbidden to mention it to him or to raise it himself.

The source of this commandment is G‑d's statement (exalted be He),1 "Do not show him any mercy."

The explanation of this is brought, "The verse,2 'Do not show him any mercy,' means that you may not raise an argument in his favor."