The Book, is the Qur'an, and Rayb means doubt. As-Suddi said that Abu Malik and Abu Salih narrated from Ibn `Abbas, and Murrah Al-Hamadani narrated from Ibn Mas`ud and several other Companions of the Messenger of Allah that,

﴿لاَ رَيْبَ فِيهِ﴾

(In which there is no Rayb), means about which there is no doubt. Abu Ad-Darda', Ibn `Abbas, Mujahid, Sa`id bin Jubayr, Abu Malik, Nafi` `Ata', Abu Al-`Aliyah, Ar-Rabi` bin Anas, Muqatil bin Hayyan, As-Suddi, Qatadah and Isma`il bin Abi Khalid said similarly. In addition, Ibn Abi Hatim said, "I do not know of any disagreement over this explanation.'' The meaning of this is that the Book, the Qur'an, is without a doubt revealed from Allah. Similarly, Allah said in Surat As- Sajdah,

﴿الم - ذَلِكَ الْكِتَابُ لاَ رَيْبَ فِيهِ هُدًى لِّلْمُتَّقِينَ ﴾

(Alif Lam Mim). The revelation of the Book (this Qur'an) in which there is no doubt, is from the Lord of all that exists) (2:1-2).

Some scholars stated that this Ayah - 2:2 - contains a prohibition meaning, "Do not doubt the Qur'an.'' Furthermore, some of the reciters of the Qur'an pause upon reading,

﴿لاَ رَيْبَ﴾

(there is no doubt) and they then continue;

﴿فِيهِ هُدًى لِّلْمُتَّقِينَ﴾

(in which there is guidance for the Muttaqin (the pious and righteous persons)). However, it is better to pause at,

﴿لاَ رَيْبَ فِيهِ﴾

(in which there is no doubt) because in this case,

﴿هُدًى﴾

(guidance) becomes an attribute of the Qur'an and carries a better meaning than,

﴿فِيهِ هُدًى﴾

(in which there is guidance).