I heard the Prophet ﷺ saying, "Our Lord will lay bare His Shin, and then all the Believers, men and women, will prostrate themselves before Him, but there will remain those who used to prostrate in the world for showing off and for gaining good reputation. Such people will try to prostrate (on the Day of Judgment), but their backs will be as stiff as if it is one bone (a single vertebra).  

سَمِعْتُ النَّبِيَّ ﷺ يَقُولُ: «يَكْشِفُ رَبُّنَا عَنْ سَاقِهِ فَيَسْجُدُ لَهُ كُلُّ مُؤْمِنٍ وَمُؤْمِنَةٍ وَيَبْقَى مَنْ كَانَ يَسْجُدُ فِي الدُّنْيَا رِئَاءً وَسُمْعَةً فَيَذْهَبُ لِيَسْجُدَ فَيَعُودُ ظَهْرُهُ طَبَقًا وَاحِدًا۔»  

that he (Ibn ʿAbbās) asked regarding ˹Allah's˺ statement ﷻ, 'The day the shin will be exposed' (Qalam 68:42), he said: 'If something is hidden from the Quran for you then seek it in poetry, for it is the dīwān (register) of the Arabs.'

As you've heard the poet saying: [..poetry omitted..], Ibn ʿAbbās said: 'This is the day of distress and severity.'  

أَنَّهُ سُئِلَ عَنْ قَوْلِهِ ﷻ {يَوْمَ يُكْشَفُ عَنْ سَاقٍ} [القلم 42] قَالَ: إِذَا خَفِي عَلَيْكُمْ شَيْءٌ مِنَ الْقُرْآنِ فَابْتَغُوهُ فِي الشِّعْرِ فَإِنَّهُ دِيوَانُ الْعَرَبِ۔

أَمَا سَمِعْتُمْ قَوْلَ الشَّاعِرِ [البحر الرجز] اصْبِرْ عَنَاقَ إِنَّهُ شَرٌّ بَاقٍ، قَدْ سَنَّ قَوْمَكَ ضَرْبُ الْأَعْنَاقْ؛ وَقَامَتِ الْحَرْبُ بِنَا عَنْ سَاقْ۔ قَالَ ابْنُ عَبَّاسٍ هَذَا يَوْمُ كَرْبٍ وَشِدَّةٍ