Musnad Ahmad 658:123

Yahya ibn Adam narrated to us, Isra'il narrated to us, from Abu Ishaq, from Al-Bara' ibn 'Azib, who said: The Messenger of Allah (peace and blessings of Allah be upon him) passed by a gathering of the Ansar and said: "If you insist on sitting, then guide the way, return the greeting, and help the oppressed."