The 269th prohibition is that we are forbidden from ignoring a lost object; instead, we must take it and return it to the owner.
The source of this prohibition is G‑d's statement1 (exalted be He), "[You must do the same to a donkey, an article of clothing, or anything else that your brother loses and you find;] you must not ignore it."
We have already explained2 the Mechilta's statement regarding lost objects: "We learn out [from these two verses] that one [who doesn't return a lost object] violates both a positive commandment and a prohibition." In the words of the Gemara,3 "returning a lost object constitutes a positive and negative commandment."
This prohibition is repeated in Mishneh Torah4 with a separate statement, "Do not just watch your brother's ox or sheep going astray." The Sifri says, "The verse, 'Do not just watch your brother's ox,' etc. constitutes a Biblical prohibition. The verse,5 'If you come across your enemy's ox [or donkey going astray, bring it back to him]' constitutes a positive command."
The details of this mitzvah are explained in the second chapter of tractate Bava Metzia.
