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."
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.
Tirmidhi (2910) narrated it through the route of Muhammad bin Bashshar, who said, Abu Bakr Hanafi told us, who said, Dahhak bin Uthman told us, from Ayub bin Musa, who said, I heard Muhammad bin Kaab Qurazi say, I heard Abdullah bin Masud say, and he mentioned the hadith.Tirmidhi said, "This hadith is also narrated by a different route from Ibn Masud. Abu Ahwas narrated it from Ibn Masud, some of them making it a marfu narration and others making it mawquf from Ibn Masud. This is a hasan sahih gharib hadith by this route."The narration of Abu Ahwas, the marfu version that Tirmidhi referred to, was recorded by Hakim in al-Mustadrak (1/555) with his chain from Salih bin Umar, who reported that Ibrahim Hijri informed him, from Abu Ahwas, from Abdullah, from the Prophet ﷺ, with additional wording.Hakim said, "This is a hadith with a sahih chain, though the two of them (Bukhari and Muslim) did not include it because of Salih bin Umar." Dhahabi commented on this, saying, "Salih is trustworthy, and Muslim included narrations from him, but Ibrahim bin Muslim is daif."However, this hadith was not narrated solely by Ibrahim Hijri. The hadith has multiple chains and routes, among them the chain that Tirmidhi mentioned and graded as sahih. So the hadith is sahih.