Musnad Ahmad 31:401

Ishaq ibn Isa narrated to us, he said: Malik informed us, from Abd al-Rahman ibn Abdullah, from his father, from Abu Sa’id al-Khudri, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "It is soon to be that the best wealth of a Muslim man will be sheep which he follows to the peaks of the mountains and the places where rain falls, fleeing with his religion from the tribulations."