Musannaf Ibn Abi Shayba 3:1386

Hushaym narrated to us, he said: Al-Shaybani informed us, from Al-Sha'bi, that he used to say: Recite behind the Imam in Dhuhr and 'Asr with the Opening of the Book (Al-Fatiha) and a Surah, and in the last two (rak'ahs) with the Opening of the Book.