Musnad Ahmad 937:128

Hashim narrated to us, Mubarak ibn Fudala narrated to us, from Abu Na'ama, Abdullah ibn al-Samit narrated to me, from Abu Dharr, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "O Abu Dharr, there will surely be leaders over you who will cause the prayer to die. If you encounter them, then perform the prayer at its time, and make your prayer with them a supererogatory (nafilah) prayer."