Musnad Ahmad 32:771

Abu Kamil narrated to us, Hammad narrated to us, he said: Qatadah, Thabit, and Humayd informed us, from Anas ibn Malik: "That the Prophet, peace and blessings of Allah be upon him, and Abu Bakr, and Umar, and Uthman used to begin the recitation with 'Al-hamdu lillahi rabbil-'alamin' (Praise be to Allah, Lord of the worlds)."