Musnad Abu Ya'la al-Mawsili 144:2

I prayed behind the Prophet, may Allah bless him and grant him peace, and Abu Bakr, and Umar, and Uthman, and Ali. They used to begin the recitation with "Al-hamdu lillahi rabbil-'alamin" (Praise be to Allah, Lord of the worlds), and they would recite "Maliki yawmid-din" (Master of the Day of Judgment).