Abdullah bin Umar (may Allah be pleased with them both) narrates that he saw a man praying two rak'ahs while the mu'adhdhin was calling the iqamah, so he threw a pebble at him and said: "Will you pray four rak'ahs for the morning prayer?"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4551