Musnad Ahmad 1012:16

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Mansur, from Hilal ibn Yasaf, from Abu al-Muthanna, from the son of the wife of 'Ubadah ibn al-Samit, from the Prophet, peace and blessings of Allah be upon him, who said: "There will be rulers who will be preoccupied by things, and they will delay the prayer from its time. So, perform the prayer at its time, and make your prayer with them a voluntary act."