Sahih al-Bukhari 163
Sahih al-Bukhari · Book 4: Ablutions (Wudu') · Washing both feet, and it is not sufficient to pass wet hands over the feet
Narrated `Abdullah bin `Amr:
حَدَّثَنَا مُوسَى ، قَالَ حَدَّثَنَا أَبُو عَوَانَةَ ، عَنْ أَبِي بِشْرٍ ، عَنْ يُوسُفَ بْنِ مَاهَكَ ، عَنْ عَبْدِ اللَّهِ بْنِ عَمْرٍو ، قَالَ تَخَلَّفَ النَّبِيُّ صلى الله عليه وسلم عَنَّا فِي سَفْرَةٍ سَافَرْنَاهَا، فَأَدْرَكَنَا وَقَدْ أَرْهَقْنَا الْعَصْرَ، فَجَعَلْنَا نَتَوَضَّأُ وَنَمْسَحُ عَلَى أَرْجُلِنَا، فَنَادَى بِأَعْلَى صَوْتِهِ " وَيْلٌ لِلأَعْقَابِ مِنَ النَّارِ ". مَرَّتَيْنِ أَوْ ثَلاَثًا.
The Prophet (ﷺ) remained behind us on a journey. He joined us while we were performing ablution for the `Asr prayer which was overdue and we were just passing wet hands over our feet (not washing them thoroughly) so he addressed us in a loud voice saying twice , "Save your heels from the fire."