Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 162
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 162, with English translation by Maimonides (Rambam).
לשבות ביום חמישים, שנאמר: "וקראתם בעצם היום הזה, מקרא קודש" (ויקרא כג,כא).
To rest on the fiftieth day [after Pesach], as [Leviticus 23:21] states: "And you shall proclaim a sacred holiday on that selfsame day."