Musnad Ahmad 957:3

Waki' narrated to us, Muhammad ibn Abdullah al-Shu'aythi narrated to us, from Layth ibn al-Mutawakkil, from Malik ibn Abdullah al-Khath'ami, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever’s feet become dusty in the way of Allah, Allah will forbid him to the Fire."