Musnad Ahmad 793:2

Hashim and Bahz narrated to us, they said: Sulayman ibn al-Mughirah narrated to us, from Humayd, who said: And one who heard the Bedouin narrated to me, saying: "I saw the Messenger of Allah, may Allah bless him and grant him peace, while he was praying, and he was wearing sandals made of cowhide." He said: "Then he spat to his left, and then he rubbed [the spot] where he spat with his sandal."