Sahih Bukhari | Volume 6 | Hadith Number 370

Narrated by 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 Habeas." 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...' (49:1)