Sunan an-Nasa'i 727
سنن النسائي#727Sahihأَخْبَرَنَا سُوَيْدُ بْنُ نَصْرٍ، قَالَ أَنْبَأَنَا عَبْدُ اللَّهِ، عَنْ سَعِيدٍ الْجُرَيْرِيِّ، عَنْ أَبِي الْعَلاَءِ بْنِ الشِّخِّيرِ، عَنْ أَبِيهِ، قَالَ رَأَيْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم تَنَخَّعَ فَدَلَكَهُ بِرِجْلِهِ الْيُسْرَى .
It was narrated from Abu Al-'Ala' bin Ash-Shikhir that his father said:"I saw the Messenger of Allah (ﷺ) spit and then rub it with his left foot
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 ZaiSahih Muslim
In-book reference: 8:40