Rashi on Deuteronomy 10:20/רש"י על דברים י׳:כ׳

1את ה׳ אלוהיך תירא THOU SHALT FEAR THE LORD THY GOD, and serve Him and cleave to Him; and after you possess all these qualities, then you may swear by His Name (cf. Rashi on 6:13 and Note thereon).