Musnad Ahmad 26:761

Yazid ibn Harun narrated to us, Muhammad ibn Ishaq informed us, from Abd al-Rahman ibn al-Aswad, from his father, who said: I entered upon Ibn Mas'ud, I and my uncle, during the midday heat. He said: So he established the prayer, and we stood behind him. He said: Then he took me by the hand, and he took my uncle by the hand. He said: Then he brought us forward until he placed each man of us on a side. Then he said: "This is how the Messenger of Allah, peace and blessings of Allah be upon him, used to do when they were three."