Rashi on Deuteronomy 23:24/רש"י על דברים כ״ג:כ״ד

1מוצא 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).