Charles Ellicott Commentary Ecclesiastes 5:4

"When thou vowest a vow unto God, defer not to pay it; for he hath no pleasure in fools: pay that which thou vowest." — Ecclesiastes 5:4 (ASV)

There is here a clear recognition of the passage in Deuteronomy. (Consult the passage; compare to Sirach 18:23.)

No pleasure in fools. —Compare to Isaiah 62:4.