Narrated Abu Huraira:

The Prophet () said, "Whoever performs ablution should clean his nose with water by putting the water in it and then blowing it out, and whoever cleans his private parts with stones should do it with odd number of stones."

حَدَّثَنَا عَبْدَانُ، قَالَ أَخْبَرَنَا عَبْدُ اللَّهِ، قَالَ أَخْبَرَنَا يُونُسُ، عَنِ الزُّهْرِيِّ، قَالَ أَخْبَرَنِي أَبُو إِدْرِيسَ، أَنَّهُ سَمِعَ أَبَا هُرَيْرَةَ، عَنِ النَّبِيِّ صلى الله عليه وسلم أَنَّهُ قَالَ ‏ "‏ مَنْ تَوَضَّأَ فَلْيَسْتَنْثِرْ، وَمَنِ اسْتَجْمَرَ فَلْيُوتِرْ ‏"‏‏.‏

Reference	: Sahih al-Bukhari 161
In-book reference	: Book 4, Hadith 27
USC-MSA web (English) reference	: Vol. 1, Book 4, Hadith 162