Musannaf Ibn Abi Shayba 9:274

'Abbad ibn al-'Awwam narrated to us, from Sufyan ibn Husayn, from al-Hakam, from Mujahid, from Ibn 'Abbas, who said: "It is not lawful to marry the women of the People of the Book if they are at war [with the Muslims]." Al-Hakam said: "So I narrated it to Ibrahim, and he was pleased with that."