Musnad Ahmad 3530
Musnad of Abdullah bin Mas'ood
Abu 'Ubaidah bin 'Abdullah narrated: 'Abdullah said: The Messenger of Allah (ﷺ) said: “The nutfah (sperm drop) remains in the uterus for forty days as it is without changing. Then when forty days have passed, it becomes an 'alaqah (blood clot), then a mudghair (chewed lump of flesh) for a similar length of time, then it becomes bones for a similar length of time. Then when Allah wants to give it its final shape, He sends an angel to it and the angel who is appointed in charge of it says: "O Lord, male or female? Doomed or blessed? Short or tall? With something missing or something superfluous? (What is) its provision and lifespan? Healthy or sick?' And he writes down all of that.” One of the people said: Then why should we strive if all of that is finished with? He said: "Strive, for each one will be helped to do that for which he was created."