The 239th prohibition is that we are forbidden from ourselves taking an object as security from someone who owes us money. It may be done only upon order from a judge and through his emissary; we may not barge into the debtor's house and take an object as security.
The source of this prohibition is G‑d's statement1 (exalted be He), "[When you make any kind of loan to your neighbor,] do not go into his house to take something as security."
In the words of the Mishneh:2 "When a person has lent money to another, he may only take something as security in court; he may not enter his house to take his security, as the verse3 says, 'You must stand outside.' "
This prohibition is a lav she'nitak l'aseh (a prohibition with a remedial positive commandment) — i.e., G‑d's statement4 (exalted be He), "Return the security to him." This is the way it is explained in the last chapter of tractate Makkos.5 You should be aware that if he does not return it, and thereby not fulfill the remedial positive commandment, he receives lashes and must repay the value of the secured object, as explained in the last chapter of Makkos.
The details of this mitzvah are explained in the 9th chapter of tractate Bava Metzia.6
