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לא תאחר לשלמו. שְׁלוֹשָׁה רְגָלִים, וּלְמָדוּהוּ רַבּוֹתֵינוּ מִן הַמִּקְרָא (ראש השנה ד'): לא 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).