1 Samuel 1:10-12

10And she was in bitterness of soul, and prayed unto Jehovah, and wept sore. 11And she vowed a vow, and said, O Jehovah of hosts, if thou wilt indeed look on the affliction of thy handmaid, and remember me, and not forget thy handmaid, but wilt give unto thy handmaid a man-child, then I will give him unto Jehovah all the days of his life, and there shall no razor come upon his head.

12And it came to pass, as she continued praying before Jehovah, that Eli marked her mouth.