Musannaf Ibn Abi Shayba 3:1778

Abu Usama narrated to us, he said: Ubayd Allah ibn Umar narrated to us, from Nafi, from Ibn Umar, that he: When he began the prayer, he would recite "In the name of Allah, the Entirely Merciful, the Especially Merciful." And when he finished the "Al-Hamd" (Al-Fatiha), he would recite "In the name of Allah, the Entirely Merciful, the Especially Merciful."