Likutei Moharan 15:3/ליקוטי מוהר"ן ט״ו:ג׳

13. And, the source of fear is daat (holy knowledge), as is written in Etz Chaim (Shaar Rosh HaShanah 2): MaNTZPaKh is located in Daat of Z’er Anpin. As is written (Proverbs 2:5), “Then you will understand the fear of God, and daat of the Lord you will find.” And the essence of knowledge is in the heart, as in (Deuteronomy 29:3), “Yet God has not given you a heart to know.” The essence of fear is also [in the heart], as our Sages taught: Of the things which are given over to [i.e., hidden in] the heart, it is said, “And you shall fear your Lord!” (Kiddushin 32b). That is, he will know whom to fear—i.e., “To fear the glorious name” (Deuteronomy 28:58), [which is] fear of His exaltedness.