عَنْ أَبِي عُثْمَانَ عَنْ أُبَيِّ بْنِ كَعْبٍ رَضِيَ اللَّهُ عَنْهُمَا أَنَّ رَجُلًا اعْتَزَى فَأَعَضَّهُ أُبَيٌّ بِهَنِ أَبِيهِ فَقَالُوا مَا كُنْتَ فَحَّاشًا قَالَ إِنَّا أُمِرْنَا بِذَلِكَ
It is narrated from Sayyiduna Ubayy ibn Ka'b (may Allah be pleased with him) that a man was attributed (with a relation of ignorance), so he interrupted his speech by cursing the man's father's private parts. The people said: O Ubayy! You were not one to speak badly? He said: Indeed, we have been commanded to do this.