Musnad Ahmad 31:552

'Abd al-Razzaq narrated to us, Malik narrated to us, from 'Abd Allah ibn 'Abd al-Rahman, 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 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."