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." (Using translation from Bukhārī 60)
تَخَلَّفَ رَسُولُ اللهِ ﷺ فِي سَفْرَةٍ سَافَرْنَاهَا فَأَدْرَكَنَا وَقَدْ أَرْهَقَتْنَا صَلَاةُ الْعَصْرِ وَنَحْنُ نَتَوَضَّأُ فَجَعَلْنَا نَمْسَحُ عَلَى أَرْجُلِنَا فَنَادَى بِأَعْلَى صَوْتِهِ وَيْلٌ لِلْأَعْقَابِمِنَ النَّارِ مَرَّتَيْنِ أَوْ ثَلَاثًا