Language:
كتاب الوضوء
4
Ablutions (Wudu')
(26)
Chapter: To clean the private parts with odd number of stones
(26)
باب الاِسْتِجْمَارِ وِتْرًا

Narrated Abu Huraira:

Allah's Messenger (ﷺ) said, "If anyone of you performs ablution he should put water in his nose and then blow it out and whoever cleans his private parts with stones should do so with odd numbers. And whoever wakes up from his sleep should wash his hands before putting them in the water for ablution, because nobody knows where his hands were during sleep."

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ يُوسُفَ، قَالَ أَخْبَرَنَا مَالِكٌ، عَنْ أَبِي الزِّنَادِ، عَنِ الأَعْرَجِ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ إِذَا تَوَضَّأَ أَحَدُكُمْ فَلْيَجْعَلْ فِي أَنْفِهِ ثُمَّ لِيَنْثُرْ، وَمَنِ اسْتَجْمَرَ فَلْيُوتِرْ، وَإِذَا اسْتَيْقَظَ أَحَدُكُمْ مِنْ نَوْمِهِ فَلْيَغْسِلْ يَدَهُ قَبْلَ أَنْ يُدْخِلَهَا فِي وَضُوئِهِ، فَإِنَّ أَحَدَكُمْ لاَ يَدْرِي أَيْنَ بَاتَتْ يَدُهُ ‏"‏‏.‏
Reference : Sahih al-Bukhari 162
In-book reference : Book 4, Hadith 28
USC-MSA web (English) reference : Vol. 1, Book 4, Hadith 163
  (deprecated numbering scheme)