Sahih al-Bukhari 60
Sahih al-Bukhari · Book 3: Knowledge · Whoever raises his voice in (conveying) knowledge
Narrated `Abdullah bin `Amr:
حَدَّثَنَا أَبُو النُّعْمَانِ، عَارِمُ بْنُ الْفَضْلِ قَالَ حَدَّثَنَا أَبُو عَوَانَةَ ، عَنْ أَبِي بِشْرٍ ، عَنْ يُوسُفَ بْنِ مَاهَكَ ، عَنْ عَبْدِ اللَّهِ بْنِ عَمْرٍو ، قَالَ تَخَلَّفَ عَنَّا النَّبِيُّ صلى الله عليه وسلم فِي سَفْرَةٍ سَافَرْنَاهَا، فَأَدْرَكَنَا وَقَدْ أَرْهَقَتْنَا الصَّلاَةُ وَنَحْنُ نَتَوَضَّأُ، فَجَعَلْنَا نَمْسَحُ عَلَى أَرْجُلِنَا، فَنَادَى بِأَعْلَى صَوْتِهِ " وَيْلٌ لِلأَعْقَابِ مِنَ النَّارِ ". مَرَّتَيْنِ أَوْ ثَلاَثًا.
Once the Prophet (ﷺ) remained behind us in a journey. He joined us while we were performing ablution for the prayer which was over-due. We were just passing wet hands over our feet (and 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."