Musannaf Ibn Abi Shayba 3:1186

Narrated to us Abd al-A’la, from al-Jurayri, from Abu al-‘Ala, from Abu Rafi’: Umar used to recite in the morning prayer one hundred [verses] from al-Baqarah, and he would follow it with a surah from the Mathani or from the beginning of the Mufassal. And he would recite one hundred [verses] from Al ‘Imran, and he would follow it with a surah from the Mathani or from the beginning of the Mufassal.