كتاب الطهارة
2  
The Book of Purification
(8)
Chapter: Odd numbers when rinsing the nose and cleaning oneself with pebbles (istijmar)
(8)
باب الإِيتَارِ فِي الاِسْتِنْثَارِ وَالاِسْتِجْمَارِ ‏‏
Sahih Muslim 237c

Abu Huraira reported:

The Messenger of Allah (way peace he upon him) said: When anyone performs ablution he must clean his nose and when anyone wipes himself with pebbles (after answering the call of nature) he must do that odd number of times.
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنِ ابْنِ شِهَابٍ، عَنْ أَبِي إِدْرِيسَ الْخَوْلاَنِيِّ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ مَنْ تَوَضَّأَ فَلْيَسْتَنْثِرْ وَمَنِ اسْتَجْمَرَ فَلْيُوتِرْ ‏"‏ ‏.‏
Reference : Sahih Muslim 237c
In-book reference : Book 2, Hadith 29
USC-MSA web (English) reference : Book 2, Hadith 460
  (deprecated numbering scheme)