Sahih Bukhari Volume 009, Book 089, Hadith Number 326.
Narrated By Anas bin Malik : That he heard 'Umar's second speech he delivered when he sat on the pulpit on the day following the death of the Prophet 'Umar recited the Tashahhud while Abu Bakr was silent. 'Umar said, "I wish that Allah's Apostle had outlived all of us, i.e., had been the last (to die). But if Muhammad is dead, Allah nevertheless has kept the light amongst you from which you can receive the same guidance as Allah guided Muhammad with that. And Abu Bakr is the companion of Allah's Apostle He is the second of the two in the cave. He is the most entitled person among the Muslims to manage your affairs. Therefore get up and swear allegiance to him." Some people had already taken the oath of allegiance to him in the shed of Bani Sa'ida but the oath of allegiance taken by the public was taken at the pulpit. I heard 'Umar saying to Abu Bakr on that day. "Please ascend the pulpit," and kept on urging him till he ascended the pulpit whereupon, all the people swore allegiance to him.