Musnad Ahmad 26:796

Muhammad ibn Ubayd narrated to us, Muhammad—meaning Ibn Ishaq—narrated to us, from Abd al-Rahman ibn al-Aswad, from his father, who said: I and Alqamah entered upon Abdullah ibn Mas’ud at midday. When the sun inclined, he established the prayer, and we stood behind him. He took my hand and the hand of my companion and placed us on his two sides, and he stood between us. Then he said: "This is how the Messenger of Allah (peace and blessings of Allah be upon him) used to do when there were three." Then he prayed with us. When he finished, he said: "There will be leaders who will delay the prayer from its appointed times. Do not wait for them to pray it, and make the prayer with them a supererogatory (nafl) prayer."