Musnad Ahmad 25:1470

Waki’ narrated to us, Salih bin 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, so he saw me while I was praying them. He pulled me and said: "Do you want to pray the morning [prayer] as four?" It was said to Ibn Abbas: "From the Prophet, peace and blessings of Allah be upon him?" He said: "Yes."