Arabic text
عن عبد الله بن مسعود رضي الله عنه قال: قال رسول الله صلى الله عليه وسلم: «مَنْ قَرَأَ حَرْفًا مِنْ كِتَابِ اللهِ فَلَهُ بِهِ حَسَنَةٌ، وَالْحَسَنَةُ بِعَشْرِ أَمْثَالِهَا، لَا أَقُولُ {الم} حَرْفٌ، وَلَكِنْ {أَلِفٌ} حَرْفٌ، وَ{لَامٌ} حَرْفٌ، وَ{مِيمٌ} حَرْفٌ».
Translation
‘Abdullāh ibn Mas‘ūd (may Allah be pleased with him) reported: The Messenger of Allah (may Allah’s peace and blessings be upon him) said: "Whoever recites a letter from the Book of Allah will be credited with a good deed for it, and a good deed is multiplied tenfold. I am not saying that 'Alif-Lām-Mīm' is one letter, but Alif is a letter, Lām is a letter, and Mīm is a letter."
Explanation
The Prophet (may Allah's peace and blessings be upon him) informed that any Muslim who recites one letter from the Book of Allah will be credited with a good deed for it, and the reward gets multiplied for him up to tenfold. To illustrate this, he said: "I am not saying that 'Alif-Lām-Mīm' is one letter, but Alif is a letter, Lām is a letter, and Mīm is a letter." Thus, they are three and equal thirty good deeds.
Source: HadeethEnc.com