Sahih Bukhari Volume 009, Book 089, Hadith Number 325.
Narrated By '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 Apostle) 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."