Parashas Eikev · Passuk 10:20
You must revere the ETERNAL your God—whom you shall worship, to whom you shall hold fast, and by whose name you shall swear.
Torah Temimah points to Temurah 3b.
TT lemma: ובשמו תשבע.
English: “And by His name you shall swear.” What does this teach, since it is already stated in Vaetchanan? It comes for Rav’s teaching: Whence that one may swear to fulfill a commandment? As it is said, “I have sworn and I will fulfill, to observe Your righteous ordinances.”
Custom (oaths to fulfill mitzvot)
An oath may be used not only to avoid sin but to strengthen oneself in performing commandments.