Positive commandment · Mitzvat Aseh #1 of 613
To know that there is a G-d
לֵידַע שֶׁיֵּשׁ שָׁם אֱלוֹהַּ
The verse · Shemot 20:2
אָנֹכִי ה׳ אֱלֹהֶיךָ אֲשֶׁר הוֹצֵאתִיךָ מֵאֶרֶץ מִצְרַיִם מִבֵּית עֲבָדִים
“I am the Lord thy God, who have brought thee out of the land of Miżrayim, out of the house of bondage.”
The Rambam’s words
The first of the positive commandments is the mitzvah to know that there is a G-d, as [Exodus 20:2] states: "I am G-d, your Lord."
מִצְוָה רִאשׁוֹנָה מִמִּצְווֹת עֲשֵׂה, לֵידַע שֶׁיֵּשׁ שָׁם אֱלוֹהַּ, שֶׁנֶּאֱמָר: "אָנֹכִי ה' אֱלֹהֶיךָ" (שמות כ ב, דברים ה ו).
Why · Sefer HaChinuch #25
Belief in Hashem
“The root of this commandment does not need elucidation. The matter is known and revealed to all that this belief is the foundation of the religion, and that one who does not believe in it denies a fundamental principle and has no share and merit with Israel.”
Who & when
“And [it] is practiced in every place and at all times by males and females.”
Where in the Mishneh Torah
Hilchot Foundations of the Torah הלכות יסודי התורה