مِشْكَاةُ الْمَصَابِيحِ
Mishkat al-Masabih 341
Purification
How to act when relieving oneself - Section 1
وَعَنْ أَبِي هُرَيْرَةَ رَضِيَ اللَّهُ عَنْهُ قَالَ: (قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: مَنْ تَوَضَّأَ فَلْيَسْتَنْثِرْ وَمَنِ اسْتَجْمَرَ فليوتر "
Abu Huraira reported God’s messenger as saying, "When anyone performs ablution he must clear his nose, and when anyone wipes himself with stones he must make use of an odd number.” (Bukhari and Muslim.)
Mishkat al-Masabih, 341مُتَّفَقٌ عَلَيْهِ (الألباني)