Musnad Ahmad 931:1

Muhammad ibn Ja‘far narrated to us, ‘Awf narrated to us, from al-Hasan, from ‘Uti ibn Damrah, from Ubayy ibn Ka‘b: A man invoked the lineage of the Jahiliyyah (pre-Islamic era), so he (Ubayy) told him to bite his father’s private part, and he did not use a euphemism. The people looked at him, so he said to the people: "I see what is in your minds. I could not do anything but say this, for the Messenger of Allah (peace and blessings of Allah be upon him) commanded us: 'If you hear someone invoking the lineage of the Jahiliyyah, tell him to bite his father’s private part, and do not use a euphemism.'"