Musannaf Ibn Abi Shayba 3:4053

Hafs narrated to us, from Ja’far, from his father, who said: The Prophet, peace and blessings of Allah be upon him, entered the mosque, and Bilal began the iqamah. Then Ibn Buhaynah stood up to pray two rak'ahs. The Prophet, peace and blessings of Allah be upon him, struck his shoulder and said, "O son of al-Qashb, are you praying the morning prayer as four?"