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۔