Musannaf Ibn Abi Shayba 37:579

Abu Usama narrated to us, he said: Kahmas narrated to us, he said: Abdullah ibn Shaqiq narrated to me, he said: Al-Aqra’ narrated to me, he said: Umar sent for the bishop. He said: So he was asking him while I was standing over them, shading them from the sun. He said to him: "Do you find us in your Book?" He said: "Your description and your deeds." He said: "So what do you find me?" He said: "I find you a horn of iron." He said: So Umar tapped his face and said: "A horn of iron?" He said: "A trustworthy, severe one." He said: So it was as if he was pleased with that. He said: "So what do you find after me?" He said: "A truthful successor who favors his close relatives." He said: So Umar said: "May Allah have mercy on Ibn Affan." He said: "So what do you find after him?" He said: "A crack of iron." He said: And there was something in Umar’s hand that he was turning over. He said: So he threw it away and said: "O my neck!" two or three times. He said: "Do not say that, O Commander of the Faithful, for he is a Muslim successor and a righteous man, but he will be succeeded while the sword is drawn and blood is being shed." He said: Then he turned to me and said: "The prayer."