Abu Dawud Book 2, Hadith Number 478.

Narated By Abdullah al-Muharibi : The Apostle of Allah (PBUH) said: When a man stands with the intention of saying prayer, or if any of you says prayer, he should not spit before him, nor at his right side; but he should do so at his left side, if there is a place for it; or he should spit under his left foot and then rub it off.