Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 159
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 159, with English translation by Maimonides (Rambam).
לשבות בראשון של פסח, שנאמר: "וביום הראשון, מקרא קודש" (שמות יב,טז).
To rest on the first day of Pesach, as [Exodus 12:16] states: "And on the first day, it shall be a sacred holiday."