Musnad Ahmad 25:200

Ibn Numayr narrated to us, Uthman ibn Hakim narrated to us, he said: Sa'id ibn Yasar informed me, from Ibn Abbas: "That the Messenger of Allah, peace and blessings of Allah be upon him, used to recite in the Fajr [prayer] in the first rak'ah: 'We have believed in Allah and in what was revealed to us and what was revealed to Abraham' [Al-Baqarah: 136] to the end of the verse. And in the second rak'ah: 'We have believed in Allah and testify that we are Muslims' [Al-Imran: 52]."