Sunan an Nasai
The Book of Purification
Hadith #43
1.43
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