Sahih al-Bukhari 165
Sahih al-Bukhari · Book 4: Ablutions (Wudu') · The washing of heels during ablution
Narrated Muhammad Ibn Ziyad:
حَدَّثَنَا آدَمُ بْنُ أَبِي إِيَاسٍ ، قَالَ حَدَّثَنَا شُعْبَةُ ، قَالَ حَدَّثَنَا مُحَمَّدُ بْنُ زِيَادٍ ، قَالَ سَمِعْتُ أَبَا هُرَيْرَةَ ـ وَكَانَ يَمُرُّ بِنَا وَالنَّاسُ يَتَوَضَّئُونَ مِنَ الْمِطْهَرَةِ ـ قَالَ أَسْبِغُوا الْوُضُوءَ فَإِنَّ أَبَا الْقَاسِمِ صلى الله عليه وسلم قَالَ " وَيْلٌ لِلأَعْقَابِ مِنَ النَّارِ " .
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.' "