1 Samuel 1

"Hannah was in deep anguish, crying bitterly as she prayed to the LORD. 11And she made this vow: 'O LORD of Heaven’s Armies, if you will look upon my sorrow and answer my prayer and give me a son, then I will give him back to you. He will be yours for his entire lifetime, and as a sign that he has been dedicated to the LORD, his hair will never be cut.'" v.10-11
Be careful what you promise in prayer because God may take you up on it. Hannah so desperately wanted a child that she was willing to strike a bargain with God. God took her up on her promise, and to Hannah's credit, she did her part, even though it was painful.

Although we are not in a position to barter with God, He may still choose to answer a prayer that has an attached promise. When you pray, ask yourself, will I follow through on any promises I make to God if He grants my request? It is dishonest and dangerous to ignore a promise, especially to God. God keeps his promises, and He expects us to keep ours.

Taken from the NLT Life Application Study Bible