From Sahih Bukhari (Hadith 3300):
Narrated Abu Sa`id al-Khudri:Allah's Messenger (ﷺ) said, "There will come a time when the best property of a man will be sheep which he will graze on the tops of mountains and the places where rain falls (i.e. pastures) escaping to protect his religion from afflictions
Reference: Bukhari 3300