Musannaf Ibn Abi Shayba 33:210

Yazid ibn Harun narrated to us, he said: Muhammad ibn ‘Amr informed us, from Muhammad ibn Ibrahim, who said: When "Indeed, those who lower their voices before the Messenger of Allah—those are the ones whose hearts Allah has tested for righteousness" was revealed, Abu Bakr al-Siddiq said: "O Messenger of Allah, may Allah bless him and grant him peace, I will not speak to you except as one speaks to a confidant, until I meet Allah."