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 one of you stands in prayer, he must not spit in front of him, nor to his right, but to his left if it is clear, or under his left foot."
Hadith Referenceالمعجم الكبير للطبراني / باب الطاء / 8137