Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 54
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 54, with English translation by Maimonides (Rambam).
לשמוח ברגלים, שנאמר: "וְשָׂמַחְתָּ בְּחַגֶּךָ" (דברים טז, יד).
To rejoice on the festivals, as [Deuteronomy 16:14] states: "And you shall rejoice on your festivals."