Sahih Muslim 554 a
Abdullah b. Shakhkhir reported on the authority of his father that he said:
I said prayer with the Messenger of Allah (ﷺ) and saw him spitting and rubbing it off with his shoe.
حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ مُعَاذٍ الْعَنْبَرِيُّ، حَدَّثَنَا أَبِي، حَدَّثَنَا كَهْمَسٌ، عَنْ يَزِيدَ بْنِ عَبْدِ اللَّهِ بْنِ الشِّخِّيرِ، عَنْ أَبِيهِ، قَالَ صَلَّيْتُ مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم فَرَأَيْتُهُ تَنَخَّعَ فَدَلَكَهَا بِنَعْلِهِ ‏.‏