(a) Tariq bin Abdullah al-Muharibi (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said to me: "When you are praying, do not spit in front of you or to your right; if your left side is clear, spit towards it, or under your foot."
(b) He said "with his foot", meaning under his leg, and that he should rub it with his foot.
(c) It is narrated from Mansur: Let him spit under his left foot.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3598