Musnad Ahmad 25:207

Ya'la narrated to us, Uthman narrated to us, from Sa'id, from Ibn Abbas, who said: "The most that the Messenger of Allah (peace and blessings of Allah be upon him) used to pray were the two rak'ahs before the dawn: 'We have believed in Allah and in what was revealed to us and what was revealed to Abraham and Ishmael' [Al-Baqarah: 136] to the end of the verse. And the other: 'We have believed in Allah and be witness that we are Muslims' [Al-Imran: 52]."