Sahih Muslim 2643b

Ibn Mas‘ūd reported: The Messenger of Allah (may Allah's peace and blessings be upon him) related to us, and he is the truthful one in his speech, and the trusted one, as Allah Almighty endorsed him.

He said: The creation of each one of you is brought together. When a man copulates with his wife, his scattered semen is brought together in the woman's womb as a sperm-drop for forty days. Then, he becomes a clinging clot for another forty days. Then, he becomes a lump, which is a piece of chewable flesh, for a third period of forty days.

Then, Allah sends to him the angel, who breathes life into him after the end of the third period. The angel is commanded to write four things: his sustenance, which is the amount of bliss he will gain during his lifetime; his lifespan, which is the duration of his stay in this world; and his deeds, as to what they will be; and whether he will be miserable or happy.

Then, the Prophet (may Allah's peace and blessings be upon him) swore that a person might perform the righteous deeds of the people of Paradise, as they outwardly seem, and he continues in this state until there is only an arm's length (a cubit) between him and Paradise; that is, only a cubit separates him from reaching Paradise.

But then destiny overtakes him, and thereupon he performs the deeds of the people of Hellfire, and they become his last deeds, and so he enters Hellfire. This is because it is required for a person's virtuous deeds to be accepted and that he continues to do them and not change course. There is another type of people, who perform the deeds of the people of Hellfire until they are close to entering it, as if only a cubit is separating between them.

But then destiny overtakes them, and they begin to perform the deeds of Paradise, and they enter it.