Mishkat al-Masabih 2137
Ibn Mas'ūd reported God’s messenger as saying:
“If anyone recites a letter of God’s Book he will be credited with a good deed, and a good deed gets a tenfold reward (Al-Qur’ān, 6:160). I do not say that A.L.M are one letter*, but alif is a letter, lām is a letter and mīm is a letter.” 
*These letters occur at the beginning of sūras 2, 3, 29, 30, 31 and 32. No satisfactory explanation of their meaning has been given. A brief discussion of them is given by Abdullāh Yūsuf Ali in The Holy Qur’ān, Text, Translation & Commentary (Lahore, 1934 and later editions), note 25. Sea also pp. 118-120.

Tirmidhī and Dārimī transmitted it, Tirmidhī saying this is a hasan sahīh tradition whose isnad is gharīb .
وَعَنِ ابْنِ مَسْعُودٍ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: 
" مَنْ قَرَأَ حَرْفًا مِنْ كِتَابِ اللَّهِ فَلَهُ بِهِ حَسَنَةٌ وَالْحَسَنَةُ بِعَشْرِ أَمْثَالِهَا لَا أَقُولُ: آلم حَرْفٌ. أَلْفٌ حَرْفٌ وَلَامٌ حَرْفٌ وَمِيمٌ حَرْفٌ ". رَوَاهُ التِّرْمِذِيُّ وَالدَّارِمِيُّ وَقَالَ التِّرْمِذِيُّ هَذَا حَدِيثٌ حَسَنٌ صَحِيحٌ غَرِيب إِسْنَادًا