The Book of Purification
1/39. Chapter:
Allowing The Usage Of One Stone For Cleaning
Sunan an-Nasai
أَخْبَرَنَا إِسْحَاقُ بْنُ إِبْرَاَهِيمَ، قَالَ أَنْبَأَنَا جَرِيرٌ، عَنْ مَنْصُورٍ، عَنْ هِلاَلِ بْنِ يِسَافٍ، عَنْ سَلَمَةَ بْنِ قَيْسٍ، عَنْ رَسُولِ اللَّهِ صلى الله عليه وسلم قَالَ " إِذَا اسْتَجْمَرْتَ فَأَوْتِرْ " .
It was narrated from Salamah bin Qais that the Messenger of Allah (ﷺ) said: "When you clean yourselves (with stones, after defecating), use an odd number."
Reference : Sunan an-Nasa'i 43In-book reference : Book 1, Hadith 43English translation : Vol. 1, Book 1, Hadith 43