Musannaf Ibn Abi Shayba 9:1563

Mu'tamir ibn Sulayman narrated to us, from Ma'mar, from 'Amr ibn Muslim, from Tawus, that the Messenger of Allah (may Allah bless him and grant him peace) commanded a herald during a military expedition he undertook, that men should not have intercourse with a pregnant woman until she delivers, nor with a woman who has conceived until she menstruates.