Musannaf Ibn Abi Shayba 3:1752

Hushaym narrated to us, he said: Humayd informed us, from Anas, that Abu Bakr, Umar, and Uthman used to begin the recitation with "Al-hamdu lillahi rabbil-'alamin" (Praise be to Allah, Lord of the worlds). Humayd said: And I think he mentioned the Prophet, may Allah bless him and grant him peace.