Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 269
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 269, with English translation by Maimonides (Rambam).
שלא יתעלם מן האבידה, שנאמר "לֹא תוּכַל לְהִתְעַלֵּם" (דברים כב, ג).
Not to ignore a lost object, as [Deuteronomy 22:3] states: "You may not ignore it."