1 Samuel 1:4-5, 11

On the day when Elkanah sacrificed, he would give portions to Peninnah his wife and to all her sons and daughters. But to Hannah he gave a double portion, because he loved her, though the LORD had closed her womb.

And she vowed a vow and said, “O LORD of hosts, if you will indeed look on the affliction of your servant and remember me and not forget your servant, but will give to your servant a son, then I will give him to the LORD all the days of his life, and no razor shall touch his head.”