Charles Ellicott Commentary Deuteronomy 23:21

"When thou shalt vow a vow unto Jehovah thy God, thou shalt not be slack to pay it: for Jehovah thy God will surely require it of thee; and it would be sin in thee." — Deuteronomy 23:21 (ASV)

When thou shalt vow ... thou shalt not be slack ... —The three yearly feasts are mentioned by Rashi and the Rabbis as occasions for the payment of vows. (See 1 Samuel 1:21.) This precept is cited in Ecclesiastes 5:4, but with sufficient verbal variation to prevent its being called a quotation.