Musnad Abu Ya'la al-Mawsili 115:40

Hadaba narrated to us, Hammam narrated to us, Qatada narrated to us, from Anas, that the Messenger of Allah (peace be upon him), Abu Bakr, and Umar used to begin the recitation with "Al-hamdu lillahi rabbil-'alamin" (Praise be to Allah, Lord of the worlds).