Musnad Ahmad 931:2

Yahya ibn Sa'id narrated to us, 'Awf narrated to us, from al-Hasan, from 'Utiy, from Ubayy ibn Ka'b, who said: I saw a man who invoked the invocation of Jahiliyyah (pre-Islamic ignorance) in the presence of Ubayy, boasting of his father. So he (Ubayy) told him to bite his father's penis, and he did not use a euphemism. Then he said to them: "Indeed, I see what is in your souls; I cannot do otherwise. I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'Whoever invokes the invocation of Jahiliyyah, then tell him to bite his father's penis, and do not use a euphemism.'"