Musannaf Ibn Abi Shayba 3:6352

Hafs ibn Ghiyath narrated to us, from Ibn Jurayj, from Ibn Abi Mulaykah, from Umm Salamah, who said: The Messenger of Allah (may Allah bless him and grant him peace) used to recite "In the name of Allah, the Most Gracious, the Most Merciful. All praise is due to Allah, Lord of the worlds," meaning letter by letter.