Sa'd al-Jari, or Abdullah bin Sa'd, the freed slave of Sayyiduna Umar (may Allah be pleased with him), said that Sayyiduna Umar (may Allah be pleased with him) said: "The Christians of the Arabs are not People of the Book, and their slaughtered animals are not lawful for us; either they become Muslim or we will kill them."
Imam al-Shafi'i (may Allah have mercy on him) said: How could we force them into Islam or kill them? For the Messenger of Allah (peace and blessings be upon him) took jizyah from the Christians of the Arabs, and Sayyiduna Umar and Sayyiduna Uthman (may Allah be pleased with them both) and Sayyiduna Ali (may Allah be pleased with him) let them remain, even though Sayyiduna Umar (may Allah be pleased with him) had said this. For that reason, marrying their women is not lawful for us, because Allah, the Lord of Might, has made marriage lawful for us only with those People of the Book upon whom the Book was revealed.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجزية / 18798