Prayers (Salat)
حَدَّثَنَا آدَمُ، قَالَ حَدَّثَنَا شُعْبَةُ، قَالَ حَدَّثَنَا قَتَادَةُ، قَالَ سَمِعْتُ أَنَسَ بْنَ مَالِكٍ، قَالَ قَالَ النَّبِيُّ صلى الله عليه وسلم " الْبُزَاقُ فِي الْمَسْجِدِ خَطِيئَةٌ، وَكَفَّارَتُهَا دَفْنُهَا ".Narrated Anas bin Malik:The Prophet (ﷺ) said, "Spitting in the mosque is a sin and its expiation is to bury it
From Sahih al-Bukhari, whose reports the scholars accept as authentic.