Musnad Ahmad 31:264

Ibn Numayr narrated to us, Yahya bin Sa’id narrated to us, from ‘Abdullah bin ‘Abd al-Rahman al-Ansari, from his father, that he heard Abu Sa’id al-Khudri saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "It is soon to be that the best of a Muslim’s wealth will be sheep which he follows to the mountain peaks and the places where rain falls, fleeing with his religion from the tribulations."