Sahih al-Bukhari 96
Narrated `Abdullah bin `Amr:
Once Allah's Messenger (ﷺ) remained behind us in a journey. He joined us while we were performing 
ablution for the `Asr prayer which was over-due. We were just passing wet hands over our feet (not 
washing them properly) so the Prophet (ﷺ) addressed us in a loud voice and said twice or thrice, "Save 
your heels from the fire."
حَدَّثَنَا مُسَدَّدٌ، قَالَ حَدَّثَنَا أَبُو عَوَانَةَ، عَنْ أَبِي بِشْرٍ، عَنْ يُوسُفَ بْنِ مَاهَكَ، عَنْ عَبْدِ اللَّهِ بْنِ عَمْرٍو، قَالَ تَخَلَّفَ رَسُولُ اللَّهِ صلى الله عليه وسلم فِي سَفَرٍ سَافَرْنَاهُ فَأَدْرَكَنَا وَقَدْ أَرْهَقْنَا الصَّلاَةَ صَلاَةَ الْعَصْرِ وَنَحْنُ نَتَوَضَّأُ، فَجَعَلْنَا نَمْسَحُ عَلَى أَرْجُلِنَا، فَنَادَى بِأَعْلَى صَوْتِهِ ‏
"‏ وَيْلٌ لِلأَعْقَابِ مِنَ النَّارِ ‏"‏‏.‏ مَرَّتَيْنِ أَوْ ثَلاَثًا‏.‏