Commentary for Deuteronomy 23:24

מוצא שפתיך תשמר THAT WHICH HATH GONE OUT OF THY LIPS THOU SHALT KEEP — This is intended to add a positive command to the prohibition of delaying one’s vows (mentioned in the previous verse) (Rosh Hashanah 6a).

מוצא שפתיך תשמר, "You shall carry out what you have promised with your lips." The Torah suggests that you should wait before making a vow, a promise, until you are in a position to honour it forthwith. In that event it is perfectly acceptable for you to make vows. This is why the Torah writes immediately adjoining ועשית כאשר נדרת, "and you will do in accordance with what you have vowed." The famous teacher of the Mishnah Hillel conducted himself according to this principle by not designating the lamb he would offer as a passover offering as such until he had reached the Temple yard where it was to be slaughtered (compare Pessachim 86).