Musnad Abu Ya'la al-Mawsili 109:277

Sulayk al-Ghatafani came while the Prophet, peace and blessings of Allah be upon him, was delivering the sermon. The Messenger of Allah, peace and blessings of Allah be upon him, said: "When one of you comes while the Imam is delivering the sermon, let him pray two rak'ahs and make them brief."