Musnad Ahmad 5:803

'Affan narrated to us, Hammad ibn Salamah narrated to us, Muhammad ibn Ishaq narrated to us, from Muhammad ibn Ibrahim al-Taymi, from Salamah ibn Abi al-Tufayl, from Ali ibn Abi Talib—may Allah be pleased with him—that the Prophet, peace and blessings of Allah be upon him, said to him: "O Ali, you have a treasure in Paradise, and you are the possessor of its two horns. So do not follow a glance with a glance, for you have the first, and you do not have the last."