Numbers 30:8

‘Or if a person swears thoughtlessly with his lips to do evil or to do good, in whatever matter a man may speak thoughtlessly with a sworn oath, and it is hidden from him, and then he comes to know it, he will be guilty in one of these.

And she made a vow and said, “O Yahweh of hosts, if You will indeed look on the affliction of Your maidservant and remember me and not forget Your maidservant, but will give Your maidservant a seed amongst men, then I will give him to Yahweh all the days of his life, and a razor shall never come on his head.”