Hazrat Umar (radhiyallahu ‘anhu) had once reached the borders of Shaam (Syria) when he was informed of a plague which had struck the people of Syria.
Hazrat Umar (radhiyallahu ‘anhu) said, “If death comes to me while Abu ‘Ubaidah bin Jarraah (radhiyallahu ‘anhu) is still alive, then I will appoint him as the khalifah after me, and if Allah Ta’ala had to ask me, ‘Why did you appoint Abu Ubaidah (radhiyallahu ‘anhu) as the khalifah to rule over the Muslims (after you)?’, I will say to Allah Ta’ala the reason I appointed him is that I had once heard Rasulullah (sallallahu alaihi wasallam) mention, “Every Nabi had a special trustworthy follower (from his ummah) and my special trustworthy follower (over my ummah) is Abu Ubaidah bin Jarraah (radhiyallahu ‘anhu).” (Musnad Ahmad #108)
It is reported that when Hazrat Umar (radhiyallahu ‘anhu) was about to pass away, he formed a group of six Sahaabah (radhiyallahu ‘anhum) and instructed that they should decide among themselves who will be appointed as the next khaleefah. At that time, Hazrat Umar (radhiyallahu ‘anhu) also said, “Had Abu Ubaidah been alive today, I would have certainly appointed him as the khalifah (of the muslims). (Tafseer Ibnu Katheer 8/54)