Sahih Muslim 237c
Sahih Muslim · Book 2: The Book of Purification · Odd numbers when rinsing the nose and cleaning oneself with pebbles (istijmar)
Abu Huraira reported:
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 " .
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.