It is narrated from Sayyiduna Malik bin al-Huwayrith (may Allah be pleased with him); he said that the Messenger of Allah (peace and blessings be upon him) said: "When Allah intends to create a soul, and the man has intercourse with the woman, his fluid spreads into every vein and sinew of hers. Then, when the seventh day comes, Allah, the Mighty and Majestic, makes green (alive) for it every vein that reaches back to Adam (peace be upon him)." Then he (peace and blessings be upon him) recited this verse: "In whatever form He willed, He assembled you." [Surah al-Infitar: 8]
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16693