Language:
كتاب الوضوء
4
Ablutions (Wudu')
(29)
Chapter: The washing of heels during ablution
(29)
باب غَسْلِ الأَعْقَابِ
وَكَانَ ابْنُ سِيرِينَ يَغْسِلُ مَوْضِعَ الْخَاتَمِ إِذَا تَوَضَّأَ.

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 ‏"‏‏.‏
Reference : Sahih al-Bukhari 165
In-book reference : Book 4, Hadith 31
USC-MSA web (English) reference : Vol. 1, Book 4, Hadith 166
  (deprecated numbering scheme)