Musnad Abu Ya'la al-Mawsili 129:157

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 (ﷺ), 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 (ﷺ).