Prayer (Kitab Al-Salat)
Dawud :: Book 2 : Hadith 485
Narrated Wathilah ibn al-Asqa':
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 Apostle of Allah (peace_be_upon_him) doing so.