Musannaf Ibn Abi Shayba 19:742

Abu Bakr narrated to us, he said: Ubayd Allah ibn Musa narrated to us, from al-Hasan ibn Salih, from Simak, he said: Fadl ibn Ka'b narrated to me, he said: Umar intended to stone a woman who had committed adultery while she was pregnant. Mu'adh said to him: "If you wrong her, have you considered what is in her womb? What is its sin? Why would you kill two souls for one?" So he left her until she delivered her child, then he stoned her.