Surah al-Infitar

Sayyiduna Malik bin al-Huwayrith (may Allah be pleased with him) narrates: The Prophet (peace and blessings be upon him) said: "When Allah Almighty intends to create a soul, and the man has intercourse with the woman, the man's fluid spreads into every vein and sinew of the woman. When the seventh day comes, Allah Almighty brings forth for it every vein between it and Sayyiduna Adam (peace be upon him)." Then he recited this verse: "In whatever form He willed, He assembled you."
Tabarani recorded it in all three of his books, and its narrators are thiqat (trustworthy).

Musa bin Ali narrates from his father, from his grandfather, that the Prophet (peace and blessings be upon him) asked him: "What has been born to you?" He said: "O Messenger of Allah! What else could be born to me but a boy or a girl?" The Prophet (peace and blessings be upon him) asked: "Whom does it resemble?" He said: "Whom else could it resemble but its mother or its father?" The Prophet (peace and blessings be upon him) said to him: "Stop! Do not say that. When the drop of fluid settles in the womb, Allah Almighty brings forth every person in the lineage between that drop and Sayyiduna Adam (peace be upon him) (and then it resembles one of them). Have you not read this verse in the Book of Allah: 'In whatever form He willed, He assembled you'?"
Tabarani recorded it; its chain contains a narrator, Mutahhar bin al-Haytham, who is matruk (abandoned).