Musannaf Ibn Abi Shayba 3:866

Waki’ narrated to us, from al-A’mash, from Ibrahim al-Taymi, from his father, who said: We used to pray the Fajr prayer, and our Imam would recite a Surah of two hundred [verses] while we were in our clothes. Then we would come to Ibn Mas’ud and find him in prayer.