Musnad Abu Ya'la al-Mawsili 115:290

Ubayd Allah narrated to us, Yazid bin Zuray' narrated to us, Sa'id narrated to us, Qatadah narrated to us, that Anas informed them: That the Prophet of Allah, may Allah bless him and grant him peace, and Abu Bakr, and Umar, and Uthman, used to begin the recitation in their prayer with "Al-hamdu lillahi rabbil-'alamin" (All praise is due to Allah, Lord of the worlds).