Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 1
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 1, with English translation by Maimonides (Rambam).
מִצְוָה רִאשׁוֹנָה מִמִּצְווֹת עֲשֵׂה, לֵידַע שֶׁיֵּשׁ שָׁם אֱלוֹהַּ, שֶׁנֶּאֱמָר: "אָנֹכִי ה' אֱלֹהֶיךָ" (שמות כ ב, דברים ה ו).
The first of the positive commandments is the mitzvah to know that there is a God, as [Exodus 20:2] states: "I am God, your Lord."