Musar על נדה 60:24

מוצא שפתיך תשמור ועשית כאשר נדרת לה' אלוקיך . This verse contains an allusion to the oath of loyalty to the Torah each soul is made to swear by the angel who taught it that same Torah while still in the mother's womb (Niddah 30b). The phrasing "keep your promise," presupposes that everyone has taken such a vow.