Musnad Abu Ya'la al-Mawsili 42:10

Abu Khaythama narrated to us, Sufyan narrated to us, from Ibn Abi Sa'sa'ah, from his father, from Abu Sa'id, from the Prophet (peace be upon him), who said: "It is soon that the best of a man'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."