It is narrated from Sayyiduna Anas bin Malik (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "None of you should spit in front of him or to his right, but rather to his left or under his foot."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3597