Musannaf Ibn Abi Shayba 3:1785

Ghundar narrated to us, from Shu'bah, from al-Hakam, Hammad, and Abu Ishaq, regarding a man who recites two surahs in a rak'ah: whenever he recites a surah, he begins with "In the name of Allah, the Most Gracious, the Most Merciful."