Sunan an-Nasa'i 43
سنن النسائي#43Sahihأَخْبَرَنَا 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
Scholarly Authentication
The label above is the grade most of these scholars gave — one dissenting opinion no longer speaks for all of them. If any scholar calls a narration fabricated, that verdict is shown instead, whatever the others say. Each scholar's own wording:
Abu GhuddahSahih
Al-AlbaniSahih
Zubair Ali ZaiIsnaad Sahih
In-book reference: 1:43