Musnad Ahmad 5:340

Abdullah narrated to us, Uthman ibn Muhammad ibn Abi Shaybah narrated to me, Ibn Fudayl narrated to us, from al-A’mash, from al-Hakam, from Abd al-Rahman ibn Abi Layla, from Ali, who said: The Prophet, peace and blessings of Allah be upon him, said: "Whoever narrates a hadith from me, while he thinks that it is a lie, then he is one of the two liars."