Musnad Ahmad 258:5

Hashim narrated to us, he said: Abu Fadalah al-Faraj narrated to us, he said: Abu Sa’d narrated to us, he said: I saw Wathilah ibn al-Asqa’ praying in the mosque of Damascus, and he spat under his left foot, then he rubbed it with his foot. When he finished, I said: "You are one of the companions of the Messenger of Allah, may Allah bless him and grant him peace, do you spit in the mosque?" He said: "This is how I saw the Messenger of Allah, may Allah bless him and grant him peace, doing."