Musnad Abu Ya'la al-Mawsili 152:219

Abu Khaythama narrated to us, Yazid bin Harun narrated to us, Muhammad bin Ishaq narrated to us, from Abd al-Rahman bin al-Aswad, from his father, who said: I and my uncle entered upon Abdullah bin Mas'ud during the midday heat. He established the prayer, so we stood behind him. He took the hand of one of us with his right hand and the other with his left hand, and he placed us to his right and to his left. When he finished praying, he said: "This is how the Messenger of Allah, may Allah bless him and grant him peace, used to do when there were three of us."