Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 80
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 80, with English translation by Maimonides (Rambam).
שלא לפסוע על המזבח, שנאמר "וְלֹא תַעֲלֶה בְּמַעֲלוֹת עַל מִזְבְּחִי" (שמות כ, כב).
Not to ascend to the altar using steps, as [Exodus 20:23] states: "Do not ascend My altar with steps."