Rashi on Deuteronomy 23:22/רש"י על דברים כ״ג:כ״ב

1לא תאחר לשלמו [WHEN THOU SHALT VOW A VOW UNTO THE LORD THY GOD] THOU SHALT NOT DELAY TO PAY IT — beyond three festivals after the vow has been made. Our Rabbis have deduced it (the fact that one does not transgress the prohibition before three festivals elapse) from a Scriptural text (Rosh Hashanah 4b).