Musannaf Ibn Abi Shayba 3:1771

Ishaq ibn Sulayman al-Razi narrated to us, from Abu Sinan, from Hammad, from Ibrahim, from al-Aswad, who said: I prayed behind Umar seventy prayers, and he did not recite "In the name of Allah, the Most Gracious, the Most Merciful" aloud in them.