Musannaf Ibn Abi Shayba 9:1564

Abu Usama narrated to us, from Abd al-Rahman ibn Yazid, who said: Al-Qasim and Makhul narrated to us, from Abu Umamah, that the Messenger of Allah (may Allah bless him and grant him peace) forbade on the day of Khaybar that pregnant women be had intercourse with until they deliver.