Sunan Abi Dawud Book 2, Hadith 93
“He then rubbed it with his shoe."
I came to the Messenger of Allah(ﷺ) who was saying prayer. He spat beneath his left foot.
AbuSa'id said: I saw Wathilah ibn al-Asqa' in the mosque of Damascus. He spat at the mat and then rubbed it with his foot. He was asked: Why did you do so? He said: Because I saw the Messenger of Allah (ﷺ) doing so.