Musnad Ahmad 32:1393

Abu al-Mughirah narrated to us, al-Awza'i narrated to us, he said: Qatadah wrote to me, Anas ibn Malik narrated to me, he said: "I prayed behind the Messenger of Allah (peace and blessings of Allah be upon him), and Abu Bakr, and Umar, and Uthman. They used to begin the recitation with 'Al-hamdu lillahi rabbil-'alamin' (Praise be to Allah, Lord of the worlds). They would not mention 'Bismillahir-rahmanir-rahim' (In the name of Allah, the Most Gracious, the Most Merciful) at the beginning of the recitation, nor at the end of it."