Musannaf Ibn Abi Shayba 3:1768

Waki’ narrated to us, from Hisham al-Dastawa’i, from Qatada, from Anas, that the Messenger of Allah (may Allah bless him and grant him peace), Abu Bakr, Umar, and Uthman used to begin the prayer with "Al-hamdu lillahi rabbil-'alamin" (Praise be to Allah, Lord of the worlds).