Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 5
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 5, with English translation by Maimonides (Rambam).
לְהִתְפַּלַּל לוֹ, שֶׁנֶּאֱמָר: "וַעֲבַדְתֶּם, אֵת ה' אֱלֹהֵיכֶם" (שמות כג, כה) – עֲבוֹדָה זוֹ תְּפִלָּה.
To pray, as [Exodus 23:25] states: "And you shall serve God, your Lord." This service is prayer.