Deuteronomy 10:20

Thou shalt fear the Lord thy God,.... Which includes the whole worship of him, external and internal:

him shalt thou serve; heartily and sincerely, according to his revealed will, and him only:

and to him shall thou cleave; and not turn aside to follow other gods, and worship them:

and swear by his name; and his only, whenever it is necessary to take an oath, which should not be done rashly or on trivial accounts; and never by any creature, but by the living God; see Deuteronomy 6:13.