Musnad Abu Ya'la al-Mawsili 1:36

Al-Qawariri narrated to us, Abu Ahmad al-Zubayri narrated to us, from Umar bin Sa'id bin Abi Husayn, from Ibn Abi Mulaykah, from Uqbah bin al-Harith, who said: I went out with Abu Bakr al-Siddiq from the Asr prayer, a few nights after the death of the Messenger of Allah, may Allah bless him and grant him peace, and Ali was walking by his side. He passed by al-Hasan bin Ali while he was playing with the boys. Abu Bakr carried him upon his shoulder and began saying: "By my father, he resembles the Prophet, he does not resemble Ali." And Ali was laughing.