Musnad Ahmad 32:944

Yahya narrated to us, Hisham narrated to us, Qatadah narrated to us, from Anas ibn Malik: "That the Messenger of Allah, 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' (All praise is due to Allah, Lord of the worlds)."