Musannaf Ibn Abi Shayba 22:22

Zayd ibn al-Hubab narrated to us, from Musa ibn Ubaydah, who said: Muhammad ibn Ka'b narrated to us, from Awf ibn Malik al-Ashja'i, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever reads a letter from the Book of Allah, Allah writes for him a good deed. I do not say 'Alif-Lam-Mim' is a letter, but the letters are separated: an Alif, a Lam, and a Mim."