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Narrated `Abdullah bin `Amr:

Allah's Messenger (ﷺ) said. "It is one of the greatest sins that a man should curse his parents." It was asked (by the people), "O Allah's Messenger (ﷺ)! How does a man curse his parents?" The Prophet (ﷺ) said, "'The man abuses the father of another man and the latter abuses the father of the former and abuses his mother."

حَدَّثَنَا أَحْمَدُ بْنُ يُونُسَ، حَدَّثَنَا إِبْرَاهِيمُ بْنُ سَعْدٍ، عَنْ أَبِيهِ، عَنْ حُمَيْدِ بْنِ عَبْدِ الرَّحْمَنِ، عَنْ عَبْدِ اللَّهِ بْنِ عَمْرٍو ـ رضى الله عنهما ـ قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ إِنَّ مِنْ أَكْبَرِ الْكَبَائِرِ أَنْ يَلْعَنَ الرَّجُلُ وَالِدَيْهِ ‏"‏‏.‏ قِيلَ يَا رَسُولَ اللَّهِ وَكَيْفَ يَلْعَنُ الرَّجُلُ وَالِدَيْهِ قَالَ ‏"‏ يَسُبُّ الرَّجُلُ أَبَا الرَّجُلِ، فَيَسُبُّ أَبَاهُ، وَيَسُبُّ أَمَّهُ ‏"‏‏.‏
Reference : Sahih al-Bukhari 5973
In-book reference : Book 78, Hadith 4
USC-MSA web (English) reference : Vol. 8, Book 73, Hadith 4
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