From Sahih Bukhari (Hadith 7218):
Narrated `Abdullah bin `Umar:It was said to `Umar, "Will you appoint your successor?" `Umar said, "If I appoint a Caliph (as my successor) it is true that somebody who was better than I (i.e., Abu Bakr) did so, and if I leave the matter undecided, it is true that somebody who was better than I (i.e., Allah's Messenger (ﷺ)) did so." On this, the people praised him. `Umar said, "People are of two kinds: Either one who is keen to take over the Caliphate or one who is afraid of assuming such a responsibility. I wish I could be free from its responsibility in that I would receive neither reward nor retribution I won't bear the burden of the caliphate in my death as I do in my life
Reference: Bukhari 7218