Riyad as-Salihin: The Book of Virtues – Hadith No. 320 — Ibn Mas’ud (May Allah be pleased with him) reported: The Messenger of Allah (ﷺ)… (Hadith No. 320)

Ibn Mas'ud (May Allah be pleased with him) reported: 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.".

Explanation:

Reference: Riyad as-Salihin, The Book of Virtues, Hadith No. 320