Silsilat al-Ahadith al-Sahihah

Sayyiduna Abu Hurairah and Sayyiduna Abu Sa'id Khudri (may Allah be pleased with them) narrated this hadith that the Messenger of Allah (peace and blessings be upon him) saw phlegm on the wall of the mosque, so he scraped it off with a pebble and then said: "If a person (while in prayer) needs to spit, he should not spit in the direction in front of his face nor to his right side; he should spit to his left side or under his left foot."