Abdullah bin Masud reported,

The Messenger of Allah ﷺ said, "Whoever recites a single letter from the Book of Allah earns a good deed for it, and each good deed is multiplied tenfold. I do not say that Alif Lam Mim is one letter, rather Alif is a letter, Lam is a letter, and Mim is a letter."

Azami 403

Sahih

عن عبد الله بن مسعود قال

قال رسول الله ﷺ: من قرأ حرفا من كتاب الله فله به حسنة، والحسنة بعشر أمثالها، لا أقول: آلم حرف ولكن ألف حرف، ولام حرف، وميم حرف.

الأعظمي 403صحيح
Contents
Reference
Commentary
Ayat
Collection
Jami al-Kamil
Book
Faith
Chapter
Faith in Allah عز وجل
Section
Speech of Allah, that it is heard and consists of letters and sound
Numbering
Jami al-Kamil 403 (2nd edition)
Azami 403