Sunan Abi Dawud 478
سنن أبي داود#478SahihNarrated 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
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Al-AlbaniSahih
Muhammad Muhyi Al-Din Abdul HamidSahih
Shuaib Al ArnautSahih
Zubair Ali ZaiIsnaad Sahih
In-book reference: 2:88