Riyad us Saliheen: Hadith 998

The Messenger of Allah (ﷺ) said, "Whoever recites a letter from the Book of Allah, he will be credited with a good deed, and a good deed gets a ten-fold reward. I do not say that Alif-Lam-Mim is one letter, but Alif is a letter, Lam is a letter and Mim is a letter."

[At- Tirmidhi].