The Prophet ﷺ saw expectoration in the direction of the Qibla of the mosque while he was leading the prayer, and scratched it off. After finishing the prayer, he said, "Whenever any of you is in prayer he should know that Allah is in front of him. So none should spit in front of him in the prayer."  

أَنَّهُ قَالَ رَأَى النَّبِيُّ ﷺ نُخَامَةً فِي قِبْلَةِ الْمَسْجِدِ وَهْوَ يُصَلِّي بَيْنَ يَدَىِ النَّاسِ فَحَتَّهَا ثُمَّ قَالَ حِينَ انْصَرَفَ إِنَّ أَحَدَكُمْ إِذَا كَانَ فِي الصَّلاَةِ فَإِنَّ اللَّهَ قِبَلَ وَجْهِهِ فَلاَ يَتَنَخَّمَنَّ أَحَدٌ قِبَلَ وَجْهِهِ فِي الصَّلاَةِ رَوَاهُ مُوسَى بْنُ عُقْبَةَ وَابْنُ أَبِي رَوَّادٍ عَنْ نَافِعٍ  

While the Muslims were offering the Fajr prayer, Messenger of Allah ﷺ suddenly appeared before them by living the curtain of the dwelling place of ʿAisha, looked towards the Muslims who were standing in rows. He smiled with pleasure. Abu Bakr started retreating to join the row on the assumption that the Prophet wanted to come out for the prayer. The Muslims intended to leave the prayer (and were on the verge of being put to trial), but the Prophet ﷺ beckoned them to complete their prayer and then he let the curtain fall. He died in the last hours of that day.  

بَيْنَمَا الْمُسْلِمُونَ فِي صَلاَةِ الْفَجْرِ لَمْ يَفْجَأْهُمْ إِلاَّ رَسُولُ اللَّهِ ﷺ كَشَفَ سِتْرَ حُجْرَةِ عَائِشَةَ فَنَظَرَ إِلَيْهِمْ وَهُمْ صُفُوفٌ فَتَبَسَّمَ يَضْحَكُ وَنَكَصَ أَبُو بَكْرٍ ؓ عَلَى عَقِبَيْهِ لِيَصِلَ لَهُ الصَّفَّ فَظَنَّ أَنَّهُ يُرِيدُ الْخُرُوجَ وَهَمَّ الْمُسْلِمُونَ أَنْ يَفْتَتِنُوا فِي صَلاَتِهِمْ فَأَشَارَ إِلَيْهِمْ أَتِمُّوا صَلاَتَكُمْ فَأَرْخَى السِّتْرَ وَتُوُفِّيَ مِنْ آخِرِ ذَلِكَ الْيَوْمِ