Musnad Abu Ya'la al-Mawsili 110:249

Zuhayr narrated to us, Waki’ ibn al-Jarrah narrated to us, Salih ibn Rustum narrated to us, from Ibn Abi Mulaykah, from Ibn Abbas, who said: The prayer was established, and I had not prayed the two rak'ahs. He saw me while I was praying them, so he passed by me and said, "Do you intend to pray the morning [prayer] as four?" It was said to Ibn Abbas, "The Prophet, may Allah bless him and grant him peace?" He said, "Yes."