Musnad Abu Ya'la al-Mawsili 37:1

Harun ibn Ma'ruf narrated to us, Damrah narrated to us, from Raja' ibn Abi Salamah, from Sulayman ibn Musa, who said: Malik ibn Abdullah al-Khath'ami passed by while he was in command of the people during the summer expedition in the land of the Byzantines. He saw a man leading his mount, so he said to him, "Ride, for I see your mount is exhausted." The man said, "I heard the Messenger of Allah (ﷺ) say: 'No servant's feet become dusty in the path of Allah except that Allah forbids the Fire from touching them.'" So Malik dismounted, and the people dismounted and walked. Never was a day seen with more people walking than that day.