Musnad Abu Ya'la al-Mawsili 115:350

Ubayd Allah narrated to us, Khalid narrated to us, Sa'id narrated to us, from Qatadah, from Anas ibn Malik, from the Prophet (peace and blessings of Allah be upon him), who said: "None of you should spit in front of him while in his prayer, nor to his right, for he is conversing with his Lord."