Musnad Ahmad 284:34

Waki’ narrated to us, Nafi’ ibn ‘Umar al-Jumahi narrated to us, from Ibn Abi Mulaykah, who said: The two best of men almost perished: Abu Bakr and ‘Umar. When the delegation of Banu Tamim came to the Prophet, may Allah bless him and grant him peace, one of them suggested al-Aqra’ ibn Habis al-Hanzali, brother of Banu Mujashi’, and the other suggested someone else. Abu Bakr said to ‘Umar: "You only intended to oppose me." ‘Umar said: "I did not intend to oppose you." Their voices rose in the presence of the Prophet, may Allah bless him and grant him peace, so there was revealed: "O you who have believed, do not raise your voices above the voice of the Prophet" [al-Hujurat: 2], up to His saying: "great" [al-Hujurat: 3]. Ibn Abi Mulaykah said: Ibn al-Zubayr said: "After that, ‘Umar—and he did not mention that regarding his father, meaning Abu Bakr—whenever he would speak to the Prophet, may Allah bless him and grant him peace, he would speak to him like one whispering a secret, so that he would not be heard until he asked him to repeat it."