Musannaf Ibn Abi Shayba 9:1562

Abu Usama narrated to us, from Ash'ath, from al-Hasan, who said: When Tustar was conquered, Abu Musa obtained captives. Umar wrote to him that no one should have intercourse with a woman until she has delivered, and that they should not share the Muslims in their children, for the fluid is the completion of the child.