Yahya bin Sa'id bin Dinar, the freed slave of the family of al-Zubayr, narrates: A trustworthy person informed me that on the day of Khaybar the Prophet (peace and blessings be upon him) forbade having relations with pregnant women, and he said: Would you water someone else's crop?
Abu Ya'la recorded it. I do not know the narrator Yahya, and Ibn Abi al-Zinad is da'if (weak), though he has been declared trustworthy.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب النكاح / 7602