Musannaf Ibn Abi Shayba 9:1554

Abu Khalid al-Ahmar narrated to us, from Dawud, from al-Sha'bi, who said: I said to him, "Abu Musa forbade, when Tustar was conquered, that pregnant women be had intercourse with, or that one shares with the polytheists in their children, for the fluid increases the child. Is this something he said based on his own opinion, or something he narrated from the Prophet?" He said: The Messenger of Allah (may Allah bless him and grant him peace) forbade on the day of Awtas that a pregnant woman be had intercourse with until she delivers, or a non-pregnant woman until she has completed a menstrual cycle (istibra').