Rashi on Deuteronomy 6:13/רש"י על דברים ו׳:י״ג

1ובשמו תשבע AND THOU SHALT SWEAR BY HIS NAME — If thou hast all the characteristics mentioned here, i.e. that thou reverest His name and servest Him, then thou mayest take an oath by mention of His name, for just because thou reverest His name, thou wilt be cautious with thy oath; but if not, thou shalt not so swear.