Musnad Ahmad 624:6

Aswad ibn Amir narrated to us, he said: Zuhayr informed us, from Abu Rawq al-Hamdani, that Abu al-Ghurayf narrated to them, he said: Safwan said: The Messenger of Allah (peace and blessings of Allah be upon him) sent us on an expedition. He said: "March in the name of Allah, in the way of Allah. You shall fight the enemies of Allah. Do not commit treachery, and do not kill a child." And for the traveler, there are three days and their nights to wipe over his leather socks, if he inserted his feet into them while in a state of purity. And for the resident, there is a day and a night.