Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 66
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 66, with English translation by Maimonides (Rambam).
שלא ילין הצלוב על העץ, שנאמר "לֹא תָלִין נִבְלָתוֹ עַל הָעֵץ" (דברים כא, כג).
Not to leave a corpse that was hung on the gallows, as [Deuteronomy 21:23] states: "Do not leave his body on the gallows."