Sahih Muslim 237a
Sahih Muslim · Book 2: The Book of Purification · Odd numbers when rinsing the nose and cleaning oneself with pebbles (istijmar)
Abu Huraira reported Allah's Apostle (ﷺ) as saying:
حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ ، وَعَمْرٌو النَّاقِدُ، وَمُحَمَّدُ بْنُ عَبْدِ اللَّهِ بْنِ نُمَيْرٍ ، جَمِيعًا عَنِ ابْنِ عُيَيْنَةَ ، - قَالَ قُتَيْبَةُ حَدَّثَنَا سُفْيَانُ، - عَنْ أَبِي الزِّنَادِ ، عَنِ الأَعْرَجِ ، عَنْ أَبِي هُرَيْرَةَ ، يَبْلُغُ بِهِ النَّبِيَّ صلى الله عليه وسلم قَالَ " إِذَا اسْتَجْمَرَ أَحَدُكُمْ فَلْيَسْتَجْمِرْ وِتْرًا وَإِذَا تَوَضَّأَ أَحَدُكُمْ فَلْيَجْعَلْ فِي أَنْفِهِ مَاءً ثُمَّ لْيَنْتَثِرْ " .
When anyone wipes himself with pebbles (after answering the call of nature) he must make use of an odd number and when any one of you performs ablution he must snuff in his nose water and then clean it.