Musannaf Ibn Abi Shayba 33:274

Waki’ narrated to us, he said: Iyas ibn Abi Tamimah narrated to us, he said: I heard ‘Ata’ ibn Abi Rabah say: ‘Ali ibn Abi Talib, when he would send out a military detachment, would appoint a man in charge of its affairs and advise him, saying: I advise you to fear Allah, for you must inevitably meet Him, and there is no end for you beyond Him, and He possesses the world and the Hereafter. And you must hold fast to that which draws you closer to Allah, for in that, with Allah, is a substitute for the world.