Sa'id bin Amr bin Sa'id heard his father say on the day of al-Marj: I heard my father say: I heard Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) say: Had I not heard the Messenger of Allah (peace and blessings be upon him) say: "Allah, Lord of Might, will protect the religion by means of Christians from Rabi'ah on the bank of the Euphrates," I would not have left any Arab without killing him, unless he accepted Islam.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجزية / 18646