Musnad Abu Ya'la al-Mawsili 115:139

Abu Khaythama narrated to us, Ismail bin Ulayyah narrated to us, from Sa'id bin Abi Arubah, from Qatadah, from Anas: That the Prophet (peace and blessings of Allah be upon him), Abu Bakr, and Umar used to begin [the prayer] with "Al-hamdu lillahi rabbil-'alamin" (All praise is due to Allah, Lord of the worlds).