Musannaf Ibn Abi Shayba 3:4826

Hushaym narrated to us, he said: Mansur informed us, from al-Hasan, who said: They were in doubt regarding the rising of the dawn during the time of Ibn Abbas. He said: So he ordered his mu'adhdhin, and he performed the iqamah for the prayer. Then he stepped forward and led them in prayer. He began [reciting] al-Baqarah until he completed it, then he bowed, then he prostrated. Then he stood up and began [reciting] Ali 'Imran until he completed it, then he bowed, then he prostrated. He said: And the morning had brightened for them.