Musannaf Ibn Abi Shayba 3:1753

Muhammad ibn Bishr narrated to us, he said: Sa'id narrated to us, he said: Qatadah narrated to us, from Anas, that the Messenger of Allah (may Allah bless him and grant him peace), Abu Bakr, Umar, and Uthman used to begin the recitation with "Al-hamdu lillahi rabbil-'alamin" (Praise be to Allah, Lord of the worlds).