Musannaf Ibn Abi Shayba 3:2468

'Abd al-Salam ibn Harb narrated to us, from Ibn Abi Farwah, from Abu Bakr ibn al-Munkadir, from Sa'id ibn al-Musayyib: That 'Umar saw a man praying two rak'ahs while the mu'adhdhin was performing the iqamah. He rebuked him and said, "There is no prayer while the mu'adhdhin is performing the iqamah, except for the prayer for which the iqamah is being performed."