Sayyiduna Anas bin Malik (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "When the believer is praying, he is speaking privately with his Lord, so let him never spit in front of him, nor to his right, but rather to his left or under his foot."
Abu Umar al-Hawdi narrates from Shu'bah: "But to his left or under his foot (rijl)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3596