Musnad Ahmad 5:436

Yahya—meaning Ibn Sa'id—narrated to us, from Shu'bah. And Hajjaj narrated to us, Shu'bah informed us, from Mansur—Yahya said: He said: Mansur narrated to me—from Rib'i, who said: I heard Ali saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Do not lie about me, for whoever lies about me will enter the Fire." Hajjaj said: I said to Shu'bah: "Did he (Rib'i) meet Ali?" He said: "Yes, he narrated to me from Ali," and he did not say: "I heard."