Deuteronomy 6:13

Thou shalt fear the Lord, thy God, not with slavish terror, but with reverential awe, and serve Him, which follows and flows out of the true fear of the Lord, and shalt swear by His name. Note that oaths of the right kind are not forbidden, but that they must be made in the name of the true God, for as such they are a manifestation of the worship of the heart, of the proper attitude toward the one true God, namely, as the Witness of truth and the Avenger of falsehood.