Musnad Ahmad 31:47

Sufyan narrated to us, from Ibn Abi Sa'sa', a sheikh from the Ansar, from his father, from Abu Sa'id, from the Prophet (peace and blessings of Allah be upon him): "It is soon that the best wealth of a Muslim man will be sheep which he follows to the peaks of the mountains and the places of rainfall, fleeing with his religion from the tribulations."