The Prophet ﷺ said: After the grave sins which Allah has prohibited the greatest sin is that a man dies while he has debt due from him and does not leave anything to pay it off, and meets Him with it.
عَنْ رَسُولِ اللَّهِ ﷺ أَنَّهُ قَالَ إِنَّ أَعْظَمَ الذُّنُوبِ عِنْدَ اللَّهِ أَنْ يَلْقَاهُ بِهَا عَبْدٌ بَعْدَ الْكَبَائِرِ الَّتِي نَهَى اللَّهُ عَنْهَا أَنْ يَمُوتَ رَجُلٌ وَعَلَيْهِ دَيْنٌ لاَ يَدَعُ لَهُ قَضَاءً
The Prophet ﷺ said: After the grave sins which Allah has prohibited the greatest sin is that a man dies while he has debt due from him and does not leave anything to pay it off, and meets Him with it. (Using translation from Abū Dāʾūd 3342)
عَنِ النَّبِيِّ ﷺ قَالَ إِنَّ أَعْظَمَ الذُّنُوبِ عِنْدَ اللهِ ﷻ أَنْ يَلْقَاهُ عَبْدٌ بِهَا بَعْدَ الْكَبَائِرِ الَّتِي نَهَى عَنْهَا أَنْ يَمُوتَ الرَّجُلُ وَعَلَيْهِ دَيْنٌ لَا يَدَعُ قَضَاءً
"إِنَّ أَعظم الذُّنُوبِ عِنْدِ اللَّهِ يَوْمَ الْقِيَامِةِ أَنْ يَلْقَاهُ بِهَا عَبْدٌ بَعْدَ الكبَائِر التى نهى اللَّهُ عنْهَا، أَنْ يموتَ الرَّجُلُ وعليهِ دينٌ لا يدعُ لهُ قضاءً".