The 204th mitzvah is that we are commanded to return a lost object to its owner.

The source of this commandment are G‑d's statements,1"[If you come across your enemy's ox or donkey going astray,] return it back to him," and2 "You must return them to your brother."

Our Sages said explicitly,3 "The return of a lost object is a positive mitzvah." They4 also said, "We learn out [from these two verses] that one [who doesn't return a lost object] violates both a positive commandment and a prohibition." We will explain the prohibition regarding the lost object in the proper place.5

The details of this mitzvah are explained in the second chapter of tractate Bava Metzia.