It is narrated from Sayyiduna Tariq bin Abdullah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When you are in prayer, do not spit in front of you, nor to your right; spit to your left, and if it is not clear, then under your left foot." Then he said: "Like this," and he rubbed it into the ground.
Hadith Referenceالمعجم الكبير للطبراني / باب الطاء / 8141