Musannaf Ibn Abi Shayba 22:240

Hafs narrated to us, from Ibn Jurayj, from Ibn Abi Mulaykah, from Umm Salamah, who said: The recitation of the Prophet, peace and blessings of Allah be upon him, was: "Al-hamdu lillahi rabbi al-'alamin" (All praise is due to Allah, Lord of the worlds). She then mentioned it letter by letter.