The wisdom behind mentioning these letters in the beginning of the Surahs, regardless of the exact meanings of these letters, is that they testify to the miracle of the Qur'an. Indeed, the servants are unable to produce something like the Qur'an, although it is comprised of the same letters with which they speak to each other. This opinion was mentioned by Ar-Razi in his Tafsir who related it to Al-Mubarrid and several other scholars. Al-Qurtubi also related this opinion to Al-Farra' and Qutrub. Az-Zamakhshari agreed with this opinion in his book, Al-Kashshaf. In addition, the Imam and scholar Abu Al-`Abbas Ibn Taymiyyah and our Shaykh Al-Hafiz Abu Al-Hajjaj Al-Mizzi agreed with this opinion. Al-Mizzi told me that it is also the opinion of Shaykh Al-Islam Ibn Taymiyyah. KAz-Zamakhshari said that these letters, "Were not all mentioned once in the beginning of the Qur'an. Rather, they were repeated so that the challenge (against the creation) is more daring. Similarly, several stories were mentioned repeatedly in the Qur'an, and also the challenge was repeated in various areas (i.e., to produce something like the Qur'an). Sometimes, one letter at a time was mentioned, such as Sad, Nun and Qaf. Sometimes two letters were mentioned, such as
(Ha Mim) (44:1) Sometimes, three letters were mentioned, such as,
(Alif Lam Mim (2: 1)) and four letters, such as,
(`Alif Lam Mim Ra) (13:1), and
(Alif Lam Mim Sad) (7:1).
Sometimes, five letters were mentioned, such as,
(Kaf Ha Ya `Ayn Sad) (19:1), and;
﴿حـم - عسق﴾
(Ha Mim. `Ayn Sin Qaf) (42:1-2).
This is because the words that are used in speech are usually comprised of one, two, three, four, or five letters.''
Every Surah that begins with these letters demonstrates the Qur'an's miracle and magnificence, and this fact is known by those well-versed in such matters. The count of these Surahs is twenty-nine. For instance, Allah said,
﴿الم ذَٰلِكَ الْكِتَابُ لاَ رَيْبَ فِيهِ﴾
(Alif Lam Mim) This is the Book (the Qur'an), wherein there is no doubt (2:1-2),
﴿الم - اللهُ لا إلَهَ إلاَّ هُوَ اَلْحَيُّ القَيُّومُ نَزَّلَ عَلَيْكَ الْكِتَٰـبَ بِالْحَقِّ مُصَدِّقاً لِّمَا بَيْنَ يَدَيْهِ﴾
(Alif Lam Mim. Allah! La ilaha illa Huwa (none has the right to be worshipped but He), Al-Hayyul-Qayyuum (the Ever Living, the One Who sustains and protects all that exists). It is He Who has sent down the Book (the Qur'an) to you (Muhammad ) with truth, confirming what came before it.) (3:1-3), and,
﴿المص كِتَٰـبٌ أُنزِلَ إِلَيْكَ فَلاَ يَكُن فِى صَدْرِكَ حَرَجٌ مِّنْهُ﴾
(Alif Lam Mim Sad. (This is the) Book (the Qur'an) sent down unto you (O Muhammad ), so let not your breast be narrow therefrom) (7:1-2).
Also, Allah said,
﴿الر كِتَابٌ أَنزَلْنَٰـهُ إِلَيْكَ لِتُخْرِجَ النَّاسَ مِنَ الظُّلُمَـتِ إِلَى النُّورِ بِإِذْنِ رَبِّهِمْ﴾
(Alif Lam Ra. (This is) a Book which We have revealed unto you (O Muhammad ) in order that you might lead mankind out of darkness (of disbelief and polytheism) into the light (of belief in the Oneness of Allah and Islamic Monotheism) by their Lord's leave) (14:1),
﴿الم - تَنْزِيلُ الْكِتَابِ لاَ رَيْبَ فِيهِ مِن رَّبِّ الْعَالَمينَ﴾
(Alif Lam Mim. The revelation of the Book (this Qur'an) in which there is no doubt, is from the Lord of the `Alamin (mankind, Jinn and all that exists)!) (32:1-2),
﴿حـم - تَنزِيلٌ مِّنَ الرَّحْمَٰنِ الرَّحِيمِ ﴾
(Ha Mim. A revelation from (Allah) the Most Gracious, the Most Merciful) (41:1-2), and,
﴿حـم - عسق- كَذَٰلِكَ يُوحِي إِلَيْكَ وَإِلَى اَلَّذِينَ مِن قَبْلِكَ اللهُ اَلْعَزِيزُ اَلْحَكَيمُ ﴾
(Ha Mim. `Ain Sin Qaf. Likewise Allah, the Almighty, the Wise sends revelation to you (O Muhammad ) as (He sent revelation to) those before you.) (42:1-3).
There are several other Ayat that testify to what we have mentioned above, and Allah knows best.
﴿ذَلِكَ الْكِتَابُ لاَ رَيْبَ فِيهِ هُدًى لِّلْمُتَّقِينَ ﴾
(2. That is Book in which there is no Rayb, guidance for the Muttaqin).