Musnad Abu Ya'la al-Mawsili 2:97

Ishaq ibn Isma'il and Abu Ja'far, my maternal uncle, narrated to us, they said: Yahya ibn Abi Bukayr narrated to us: 'Abdullah ibn 'Umar al-Qurashi narrated to us, he said: Sa'id ibn 'Amr ibn Sa'id narrated to me that he heard his father on the day of al-Marj saying: I heard 'Umar ibn al-Khattab saying: "Were it not that I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'Indeed, Allah protects the religion through the Christians of Rabi'ah on the bank of the Euphrates,' I would not have left a single Arab except that I would have killed him or he would have accepted Islam."