Numbers 30:2

If a man vow a vow unto the Lord, or swear an oath to bind his soul with "a bond, both the vows to perform and the vows to abstain being included in the precept, he shall not break his word and thus profane his solemn utterance, Psalms 55:20, he shall do according to all that proceedeth out of his mouth, for the promise, supported with an oath, has called upon God as a witness, and therefore its fulfillment is a sacred obligation. Far better not to make a promise than to do so lightly and afterward not keep one's word, Deuteronomy 23:21. The purpose of this regulation, which looked forward to the time of Israel's living in Canaan, was to prevent frivolous vowing and foolish promising, a practice which to this day tends to loosen the bonds of obligation which men ought to feel in their intercourse with one another.