Musannaf Ibn Abi Shayba 3:1357

Hushaym narrated to us, from Mughirah and al-Shaybani, from al-Sha’bi; and Hajjaj, from ‘Ata’; and Mansur, from al-Hasan, that they said: Recite in the two rak'ahs—meaning the latter two of the Dhuhr and the ‘Asr—with the Opening of the Book (al-Fatihah).