It was narrated from Ubayy ibn Ka‘b that a man boasted in an arrogant manner of his tribal lineage, so he told him to bite his father’s penis, and he did not use a metaphor. The people looked askance at him, so he said to the people: I can see what you are thinking, and I can only say this: that the Messenger of Allah (blessings and peace of Allah be upon him) instructed us: “If you hear someone boasting in an arrogant manner of his tribal lineage, then tell him to bite his father’s penis, and do not use a metaphor.”

Narrated by Ahmad (35/157); classed as hasan by the commentators on al-Musnad.

It was narrated from Ubayy (may Allah be pleased with him) that a man boasted of his tribal lineage, and Ubayy told him to bite his father’s penis, and they said: You were never given to obscene speech! He said: We were instructed to do that.

Narrated by Ahmad (35/142); classed as hasan by the commentators on al-Musnad and as saheeh by al-Albaani in Saheeh al-Jaami‘.

Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:

Hence some of the scholars said that this indicates that it is permissible to bluntly state the name of the private part if there is a need to do so or there is an interest to be served thereby, and this does not come under the heading of obscene speech that is forbidden, as in the hadith of Ubayy ibn Ka‘b, according to which the Prophet (blessings and peace of Allah be upon him) said:

“If you hear someone boasting in an arrogant manner of his tribal lineage, then tell him to bite his father’s penis, and do not use a metaphor.” Narrated by Ahmad. And Ubayy ibn Ka‘b heard a man saying: O So and so, and he said: Bite your father’s penis. Something was said to him about that, and he said: This is what the Messenger of Allah (blessings and peace of Allah be upon him) instructed us to do (in the case of such a person).

Minhaaj as-Sunnah an-Nabawiyyah (8/408, 409)