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."  

تَخَلَّفَ عَنَّا النَّبِيُّ ﷺ فِي سَفْرَةٍ سَافَرْنَاهَا فَأَدْرَكَنَا وَقَدْ أَرْهَقَتْنَا الصَّلاَةُ وَنَحْنُ نَتَوَضَّأُ فَجَعَلْنَا نَمْسَحُ عَلَى أَرْجُلِنَا فَنَادَى بِأَعْلَى صَوْتِهِ: «وَيْلٌ لِلأَعْقَابِ مِنَ النَّارِ!» مَرَّتَيْنِ أَوْ ثَلاَثًا۔  

I heard Abu Huraira saying as he passed by us while the people were performing ablution from a utensil containing water, "Perform ablution perfectly and thoroughly for Abul-Qasim (the Prophet) said, 'Save your heels from the Hell-fire.' "  

سَمِعْتُ أَبَا هُرَيْرَةَ وَكَانَ يَمُرُّ بِنَا وَالنَّاسُ يَتَوَضَّئُونَ مِنَ الْمِطْهَرَةِ قَالَ أَسْبِغُوا الْوُضُوءَ فَإِنَّ أَبَا الْقَاسِمِ ﷺ قَالَ وَيْلٌ لِلأَعْقَابِ مِنَ النَّارِ