Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 53
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 53, with English translation by Maimonides (Rambam).
להיראות ברגלים, שנאמר: "שָׁלֹשׁ פְּעָמִים בַּשָּׁנָה יֵרָאֶה כָּל זְכוּרְךָ" (שמות כג יז; שם לג, כג; דברים טז טז.
To appear [before God in the Temple] on the festivals, as [Deuteronomy 16:16] states: "On three occasions during the year, all your males shall appear...."