Musnad Ahmad 5:729

Abdullah narrated to us, Muhammad ibn Sulayman Luwayn narrated to us, Muhammad ibn Jabir narrated to us, from Simak, from Hanash, from Ali, who said: When ten verses of [Surah] Bara'ah were revealed to the Prophet, may Allah bless him and grant him peace, the Prophet, may Allah bless him and grant him peace, called Abu Bakr and sent him with them to recite them to the people of Mecca. Then the Prophet, may Allah bless him and grant him peace, called me and said to me: "Overtake Abu Bakr, and wherever you catch up with him, take the book from him, then go with it to the people of Mecca and recite it to them." So I caught up with him at al-Juhfah and took the book from him. Abu Bakr returned to the Prophet, may Allah bless him and grant him peace, and said: "O Messenger of Allah, has something been revealed regarding me?" He said: "No, but Gabriel came to me and said: 'None shall convey on your behalf except you or a man from you.'"