Musnad Abu Ya'la al-Mawsili 115:140

Abu Musa narrated to us, Ibn Abi 'Adi narrated to us, from Sa'id, from Qatadah, from Anas, that the Prophet (ﷺ), Abu Bakr, and 'Umar used to begin [the prayer] with "Al-hamdu lillahi rabbil-'alamin" (Praise be to Allah, Lord of the worlds).