Sahih Bukhari Volume 004, Book 055, Hadith Number 549.

Narated By Abdullah : Allah's Apostle, the true and truly inspired said, "(as regards your creation), every one of you is collected in the womb of his mother for the first forty days, and then he becomes a clot for an other forty days, and then a piece of flesh for an other forty days. Then Allah sends an angel to write four words: He writes his deeds, time of his death, means of his livelihood, and whether he will be wretched or blessed (in religion). Then the soul is breathed into his body. So a man may do deeds characteristic of the people of the (Hell) Fire, so much so that there is only the distance of a cubit between him and it, and then what has been written (by the angel) surpasses, and so he starts doing deeds characteristic of the people of Paradise and enters Paradise. Similarly, a person may do deeds characteristic of the people of Paradise, so much so that there is only the distance of a cubit between him and it, and then what has been written (by the angel) surpasses, and he starts doing deeds of the people of the (Hell) Fire and enters the (Hell) Fire."