Sa'd al-Falahah, the freed slave of Sayyiduna Umar (may Allah be pleased with him), or Ibn Sa'd al-Falahah, reports that Sayyiduna Umar (may Allah be pleased with him) said: The Christians of the Arabs are not People of the Book, nor are their slaughtered animals lawful for us, and I will not leave them until they accept Islam, or else I will strike their necks.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19169