Musnad Ahmad 4282

It was narrated from "Abdur-Rahman bin al-Aswad that his father said: My paternal uncle and I entered upon Ibn Masood at midday when it was very hot. He gave the iqamah for prayer and we stood behind him. He took hold of my hand and my uncle's hand and brought us forward until he made each of us stand on either side of him, then he said: This is what The Messenger of Allah (ﷺ) used to do if they were three.