Musannaf Ibn Abi Shaybah 4925 — Praying once the muezzin has started

Sa'id bin al-Musayyib said: Sayyiduna Umar (may Allah be pleased with him) saw a man praying two rak'ahs while the mu'adhdhin was calling the iqamah. He rebuked him and said: When the mu'adhdhin is calling the iqamah, there is no prayer except the prayer for which the iqamah is being called.