Sahih Bukhari

Narrated `Abdullah bin Az-Zubair:A group of Bani Tamim came to the Prophet (and requested him to appoint a governor for them). Abu Bakr said, ""Appoint Al-Qaqa bin Mabad."" `Umar said, ""Appoint Al-Aqra' bin Habis."" On that Abu Bakr said (to `Umar). ""You did not want but to oppose me!"" `Umar replied ""I did not intend to oppose you!"" So both of them argued till their voices grew loud. So the following Verse was revealed: 'O you who believe! Be not forward