Sunan an-Nasa'i, Volume 1, Book 8, Hadith 728 (DarusSalam) ⓘ
It was narrated from Abu Al-'Ala' bin Ash-Shikhir that his father said: "I saw the Messenger of Allah spit and then rub it with his left foot."
أَخْبَرَنَا 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 .
also referenced under: Hadith 727 (Reference) & Book 8, Hadith 40 (In-Book)