Musnad Ahmad 32:757

Ghassan ibn Mudar narrated to us, Sa'id—meaning Ibn Yazid Abu Maslamah—narrated to us, he said: I asked Anas: "Did the Prophet, peace and blessings of Allah be upon him, recite 'In the name of Allah, the Most Gracious, the Most Merciful' or 'All praise is due to Allah, Lord of the worlds'?" He said: "You are asking me about something I do not remember—or no one has asked me before you."