Musnad Ahmad 957:2

Al-Walid ibn Muslim narrated to us, Ibn Jabir narrated to us, that Abu al-Musabbah al-Awza'i narrated to them, saying: While we were traveling in the pass of Qalamiyah, the commander Malik ibn Abdullah al-Khath'ami called out to a man who was leading his horse along the side of the mountain: "O Abu Abdullah, will you not ride?" He said: "I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'Whoever’s feet become dusty in the path of Allah for an hour of the day, they are forbidden to the Fire.'"