Musnad Abu Ya'la al-Mawsili 209:42

Abu Bakr ibn Abi Shaybah narrated to us, Hafs narrated to us, from Ibn Jurayj, from Ibn Abi Mulaykah, from Umm Salamah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) used to recite "In the name of Allah, the Most Gracious, the Most Merciful," [and] "All praise is due to Allah, Lord of the worlds," meaning letter by letter.