The gravest of sins

عَنْ عَبْدِ الرَّحْمَانِ بْنِ أَبِىْ بَكْرَةَ، عَنْ أَبِيْهِ رَضِىَ اللهُ عَْنهُ قَالَ قَالَ النَّبِيُّ صَلَّى اللهُ عَلَيْه وَسَلَّمَ ‏أَلَا أُنَبِّئُكُمْ بِأَكْبَرِ الْكَبَائِرِ ثَلاَثًا‏‏ قَالُوْا بَلَى يَا رَسُوْلَ اللَّهِ‏. قَالَ ‏الْإِشْرَاكُ بِاللَّهِ وَعُقُوْقُ الْوَالِدَيْنِ‏‏.‏ وَجَلَسَ وَكَانَ مُتَّكِئًا فَقَالَ ‏أَلَا وَقَوْلُ الزُّورِ‏‏.‏ قَالَ فَمَا زَالَ يُكَرِّرُهَا حَتَّى قُلْنَا لَيْتَهُ سَكَتَ

Hazrat Abu Bakr^ra narrates that the Holy Prophet^sa said thrice, “Should I inform you of the gravest of the grave sins?” They said, “Yes, O Allah’s Messenger^sa!” He said, “Setting up equals to Allah and being disobedient and undutiful to parents.” The Prophet^sa then sat up, leaving the cushion he was leaning against, and said [with great force], “Hearken! And lying.” And he repeated this so many times that we wished he had left it off at that [and not strained himself so much.]

(Sahih al-Bukhari, Kitab al-Shahadat, Hadith 2654)