2 When a man makes a vow to the LORD or takes an oath to obligate himself by a pledge, he must not break his word but must do everything he said.

Read Numbers 30:2 Using Other Translations

If a man vow a vow unto the LORD, or swear an oath to bind his soul with a bond; he shall not break his word, he shall do according to all that proceedeth out of his mouth.
If a man vows a vow to the LORD, or swears an oath to bind himself by a pledge, he shall not break his word. He shall do according to all that proceeds out of his mouth.
A man who makes a vow to the LORD or makes a pledge under oath must never break it. He must do exactly what he said he would do.