It is narrated from Sayyiduna Tariq bin Abdullah al-Muharibi (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When you pray, do not spit in front of you, nor to your right; rather, spit to your left if it is clear, otherwise under your foot." And he (peace and blessings be upon him) gestured with his foot and scraped the ground.
Hadith Referenceالمعجم الكبير للطبراني / باب الطاء / 8134