Sahih Bukhari Volume 2, Book 23, Hadith Number 333.
Narrated Abu Salama from Ibn Abbas : Abu Bakr came out and ‘Umar , was addressing the people, and Abu Bakr told him to sit down but ‘Umar refused. Abu Bakr again told him to sit down but ‘Umar again refused. Then Abu Bakr recited the Tashah-hud (i.e. none has the right to be worshipped but Allah and Muhammad is Allah’s Apostle) and the people attended to Abu Bakr and left ‘Umar. Abu Bakr said, “Amma ba’du, whoever amongst you worshipped Muhammad, then Muhammad is dead, but whoever worshipped Allah, Allah is alive and will never die. Allah said: ‘Muhammad is no more than an Apostle and indeed (many) Apostles have passed away before him… (up to the) grateful.’ ” (3.144) (The narrator added, “By Allah, it was as if the people never knew that Allah had revealed this verse before till Abu Bakr recited it and then whoever heard it, started reciting it.”)