Sahih al-Bukhari 165
Narrated Muhammad Ibn Ziyad:
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.' "
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 ‏"‏‏.‏