حَدَّثَنَا مُسْلِمُ بْنُ إِبْرَاهِيمَ، حَدَّثَنَا هِشَامٌ، وَشُعْبَةُ، وَأَبَانُ، عَنْ قَتَادَةَ، عَنْ أَنَسِ بْنِ مَالِكٍ، أَنَّ النَّبِيَّ صلى الله عليه وسلم قَالَ ‏"‏ التَّفْلُ فِي الْمَسْجِدِ خَطِيئَةٌ وَكَفَّارَتُهُ أَنْ تُوَارِيَهُ ‏"‏ ‏.‏

Ibn ‘Umar reported

One day while the Messenger of Allah(ﷺ)was giving sermon he suddenly saw phlegm on the wall towards the qiblah(the direction to which Muslims turn in prayer) of the mosque. So he became angry at people. He then scraped it and sent for saffron and stained with it. He then said: When any one of you prays, Allah, the Exalted, faces him: he, therefore, should not spit before him.