It is narrated from Sayyiduna Tariq bin Abdullah al-Muharibi (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "When you are in prayer, do not spit in the direction of your face, nor to your right, but to your left if it is clear, otherwise under your foot."
Hadith Referenceالمعجم الكبير للطبراني / باب الطاء / 8135