Musnad Ahmad 1108:1

Muhammad ibn Ja'far and Hajjaj narrated to us, they said: 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. Hajjaj said: 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 commanders who will be occupied by things and will delay the prayer from its time. So, perform the prayer at its time, then make your prayer with them a voluntary act."