Musnad Ahmad 2:27

Abu al-Mughirah and ‘Isam ibn Khalid narrated to us, they said: Safwan narrated to us, from Shurayh ibn ‘Ubayd and Rashid ibn Sa‘d and others, they said: When ‘Umar ibn al-Khattab reached Sargh, it was reported to him that there was a severe plague in al-Sham. He said: "It has reached me that the plague is severe in al-Sham. I said: If my appointed time overtakes me while Abu ‘Ubaydah ibn al-Jarrah is alive, I will appoint him as successor. If Allah asks me: 'Why did you appoint him as successor over the nation of Muhammad, may Allah bless him and grant him peace?' I will say: 'I heard Your Messenger, may Allah bless him and grant him peace, say: "Indeed, for every prophet there is a trustworthy one, and my trustworthy one is Abu ‘Ubaydah ibn al-Jarrah."'" The people disapproved of that and said: "What about the elite of Quraysh?"—meaning the Banu Fihr. Then he said: "If my appointed time overtakes me while Abu ‘Ubaydah has passed away, I will appoint Mu‘adh ibn Jabal as successor. If my Lord, the Mighty and Majestic, asks me: 'Why did you appoint him as successor?' I will say: 'I heard Your Messenger, may Allah bless him and grant him peace, say: "He will be gathered on the Day of Resurrection in front of the scholars by a distance."'"