Sunan Abu Dawud
478
Sahih+3

Narrated Abdullah al-Muharibi: The Messenger of Allah (ﷺ) 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

SahihAl-Albani
SahihMuhammad Muhyi Al-Din Abdul Hamid