Sahih al-Bukhari 416
Narrated Abu Huraira:
Prophet said, "If anyone of you stands for prayer, he should not spit in front of him because in prayer 
he is speaking in private to Allah and he should not spit on his right as there is an angel, but he can 
spit either on his left or under his left foot and bury it (i.e. expectoration).
حَدَّثَنَا إِسْحَاقُ بْنُ نَصْرٍ، قَالَ حَدَّثَنَا عَبْدُ الرَّزَّاقِ، عَنْ مَعْمَرٍ، عَنْ هَمَّامٍ، سَمِعَ أَبَا هُرَيْرَةَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ ‏
"‏ إِذَا قَامَ أَحَدُكُمْ إِلَى الصَّلاَةِ فَلاَ يَبْصُقْ أَمَامَهُ، فَإِنَّمَا يُنَاجِي اللَّهَ مَا دَامَ فِي مُصَلاَّهُ، وَلاَ عَنْ يَمِينِهِ، فَإِنَّ عَنْ يَمِينِهِ مَلَكًا، وَلْيَبْصُقْ عَنْ يَسَارِهِ أَوْ تَحْتَ قَدَمِهِ، فَيَدْفِنُهَا ‏"‏‏.‏