Musnad Ahmad 33:290

Abu Mu'awiyah narrated to us, Al-A'mash narrated to us, from Abu Sufyan, from Jabir, who said: Sulayk al-Ghatafani came on the day of Friday while the Prophet, peace and blessings of Allah be upon him, was delivering the sermon, and he sat down. The Messenger of Allah, peace and blessings of Allah be upon him, said: "When one of you comes on the day of Friday while the Imam is delivering the sermon, let him pray two rak'ahs, then let him sit."