Musnad Abu Ya'la al-Mawsili 117:248

Zuhayr narrated to us, Affan narrated to us, Hammad narrated to us, Qatadah, Thabit, and Humayd 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 prayer with "Al-hamdu lillahi rabbil-'alamin" (Praise be to Allah, Lord of the worlds). And Humayd did not mention the Prophet (may Allah bless him and grant him peace).