Abdullah bin Rabi'ah said: We were sitting with Abdullah bin Mas'ud (may Allah be pleased with him) when the people mentioned a man and spoke about his character. Abdullah (may Allah be pleased with him) said: "What do you think, if you were to cut off his head, could you put it back?" They said: "No." He said: "Then his hand?" They said: "No." He said: "Then his foot?" They said: "No." He said: "Then you will never be able to change his character until you change his physical form. Indeed, the drop of semen settles in the womb for forty nights, then it descends as blood, then it becomes a clinging clot ('alaqah), then it becomes a lump of flesh (mudghah), then the angel is sent to it, and he writes down its provision, its character, and whether it will be wretched or fortunate."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8852