Musnad Ahmad 658:110

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from 'Adi ibn Thabit, from al-Bara', who said: I saw the Messenger of Allah, peace and blessings of Allah be upon him, placing al-Hasan ibn 'Ali upon his shoulder, while he was saying: "O Allah, I love him, so love him."