Musnad Ahmad 32:1946

Yahya ibn Sa'id narrated to us, he said: Hisham informed me, from Qatadah, from Anas: "That the Prophet, peace and blessings of Allah be upon him, and Abu Bakr, and Umar, and Uthman, used to begin the recitation with 'Al-hamdu lillahi rabbil-'alamin' [Praise be to Allah, Lord of the worlds]."