Musannaf Ibn Abi Shayba 27:375

Ibn Idris narrated to us, from Isma'il, from Zubayd, who said: When death approached Abu Bakr, he sent for Umar to appoint him as his successor. He (Umar) said: The people said, "Do you appoint over us a harsh, rough man? If he were to rule over us, he would be even harsher and rougher. What will you say to your Lord when you come to Him, having appointed him over us?" He (Abu Bakr) said: "Do you frighten me with my Lord? I will say: 'O Allah, I appointed over them the best of Your creation.'"