Positive commandment · Mitzvat Aseh #4 of 613
To fear Him
לְיִרְאָה מִמֶּנּוּ
The verse · Devarim 6:13
אֶת־ה׳ אֱלֹהֶיךָ תִּירָא וְאֹתוֹ תַעֲבֹד וּבִשְׁמוֹ תִּשָּׁבֵעַ
“Thou shalt fear the Lord thy God, and serve him, and shalt swear by his name.”
The Rambam’s words
To fear Him, as [Deuteronomy 6:13] states: "Fear G-d, your Lord."
לְיִרְאָה מִמֶּנּוּ, שֶׁנֶּאֱמָר: "אֶת-ה' אֱלֹהֶיךָ תִּירָא" (דברים ו יג, י כ).
Why · Sefer HaChinuch #432
Fearing Hashem
“The root of the commandment of fearing G-d, may He be blessed, is revealed to all who see the Sun, as the greatest protection from sin is the fear of His punishment.”
Who & when
“And this commandment is practiced in every place and at all times and by the entire human species.”
Where in the Mishneh Torah
Hilchot Foundations of the Torah הלכות יסודי התורה