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
Hadith No. 4847
Sahih al Bukhari