Sahih Bukhari Volume 001, Book 006, Hadith Number 315.
Narated By Anas bin Malik : The Prophet said, "At every womb Allah appoints an angel who says, 'O Lord! A drop of semen, O Lord! A clot. O Lord! A little lump of flesh." Then if Allah wishes (to complete) its creation, the angel asks, (O Lord!) Will it be a male or female, a wretched or a blessed, and how much will his provision be? And what will his age be?' So all that is written while the child is still in the mother's womb."