Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 202
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 202, with English translation by Maimonides (Rambam).
לעזוב מעל חברו או מעל בהמתו, שנאמר: "עזוב תעזוב, עימו" (שמות כג,ה).
To assist a colleague in unloading a burden which he or his beast [is carrying], as [Exodus 23:5] states: "You shall surely help him."b