Musannaf Ibn Abi Shayba 35:324

Hushaym narrated to us, from Abd al-Malik, from Ata’, that a man prayed the morning prayer with the Prophet, peace and blessings of Allah be upon him. When the Prophet, peace and blessings of Allah be upon him, finished the prayer, the man stood up and prayed two rak'ahs. The Prophet, peace and blessings of Allah be upon him, said to him, "What are these two rak'ahs?" He said, "O Messenger of Allah, peace and blessings of Allah be upon him, I came while you were in prayer, and I had not prayed the two rak'ahs before the dawn, so I disliked praying them while you were praying. When you finished the prayer, I stood up and prayed them." He said: So he neither commanded him nor forbade him.