Musnad Ahmad 33:974

Yazid ibn Harun narrated to us, Hisham ibn Hassan narrated to us, from al-Hasan, from Jabir ibn Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When you are in a time of plenty, let the riders have their fill of the grazing, and do not exceed the stages of the journey. When you are in a time of drought, then hasten, and you must travel by night, for the earth is folded up by night. If the ghouls (spirits) appear to you, then hasten with the Adhan. Do not pray on the main roads, and do not camp upon them, for they are the dwelling places of snakes and beasts of prey. Do not relieve yourselves upon them, for they are the places of cursing."