Sahih Muslim Book 033, Hadith Number 6397.

Chapter : The growth of a child in the womb of a mother and his destiny in regard to his livelihood, his deeds. both good and evil.

Anas b. Malik reported directly from Allah's Messenger (may peace be upon him) that he said: Allah, the Exalted and Glorious, has appointed an angel as the caretaker of the womb, and he would say: My Lord, it is now a drop of semen; my Lord, It is now a clot of blood; my Lord, it has now become a lump of flesh, and when Allah decides to give it a final shape, the angel says: My Lord, would it be male or female or would he be an evil or a good person? What about his livelihood and his age? And it is all written as he is in the womb of his mother.