Narrated Ibn Abi Mulaika:
The two righteous persons were about to be ruined. They were Abu Bakr and 'Umar who raised their voices in the presence of the Prophet when a mission from Bani Tamim came to him. One of the two recommended Al-Aqra' bin Habeas, the brother of Bani Mujashi (to be their governor) while the other recommended somebody else. (Nafi', the sub-narrator said, I do not remember his name). Abu Bakr said to Umar, "You wanted nothing but to oppose me!" 'Umar said, "I did not intend to oppose you." Their voices grew loud in that argument, so Allah revealed: 'O you who believe! Raise not your voices above the voice of the Prophet.' (49.2) Ibn Az-Zubair said, "Since the revelation of this Verse, 'Umar used to speak in such a low tone that the Prophet had to ask him to repeat his statements." But Ibn Az-Zubair did not mention the same about his (maternal) grandfather (i.e. Abu Bakr).
• Sahih al-Bukhari, Vol. 6, Book of Prophetic Commentary on the Qur'an, Hadith 368
• Sahih al-Bukhari, Book of Prophetic Commentary on the Qur'an, Hadith 368