Musnad Abu Ya'la al-Mawsili 118:83

Humaid ibn al-Rabi' al-Khazzaz narrated to us, Ruwaym al-Qari' narrated to us, al-Layth ibn Sa'd informed us, from 'Uqayl, from al-Zuhri, Anas informed me that the Prophet, peace and blessings of Allah be upon him, said: "If the land is fertile, then dismount from the backs of your mounts and give them their right of the herbage. And if the land is barren, then pass over it quickly with their marrow (i.e., while they are still strong). And you must travel by night, for the earth is traversed by night."