Maliks Muwatta Book 54, Hadith Number 16.

Malik related to me from Abd ar-Rahman ibn Abd ar-Rahman ibn Sasaca from his father that Abu Said al-Khudri said that the Messenger of Allah, may Allah bless him and grant him peace, said, “It will soon happen that the best property of a muslim will be sheep which he takes to the peaks of the mountains and the valleys, fleeing with his deen from trials.”