1 Samuel 1:10-12

10She was deeply distressed and prayed to the Lord and wept bitterly.

11And 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."

12As she continued praying before the Lord, Eli observed her mouth.