אֶת־יְהוָ֧ה אֱלֹהֶ֛יךָ תִּירָ֖א אֹת֣וֹ תַעֲבֹ֑ד וּב֣וֹ תִדְבָּ֔ק וּבִשְׁמ֖וֹ תִּשָּׁבֵֽעַ׃
Thou shalt fear the LORD thy God; Him shalt thou serve; and to Him shalt thou cleave, and by His name shalt thou swear.
Read kabbalistic (mystical) interpretation of Deuteronomy 10:20: and classical mystical sources.