Luke 1:31, 34-35

And behold, thou shalt conceive in thy womb, and bring forth a son, and shalt call his name JESUS.

And Mary said unto the angel, How shall this be, seeing I know not a man? And the angel answered and said unto her, The Holy Spirit shall come upon thee, and the power of the Most High shall overshadow thee: wherefore also the holy thing which is begotten shall be called the Son of God.