Musnad Ahmad 937:201

Isma'il narrated to us, Salih ibn Rustum narrated to us, from Abu 'Imran al-Jawni, from 'Abdullah ibn al-Samit, from Abu Dharr, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said to me: "O Abu Dharr, there will be rulers over you who will delay the prayer from its appointed times. If you encounter them, then pray the prayer at its time—and perhaps he said: in your dwelling—then go to them. If you find that they have prayed, you will have already prayed, and if you find that they have not prayed, you will have prayed with them, and it will be a supererogatory prayer for you."