Even though it is an essential doctrine of Islam, the eternal / heavenly / well-guarded Tablet is mentioned only once in the Qur'an, al-Buruj 85:21-22:
bal huwa qur'aan(un) majeed(un) fee lawH(in) maHfooZ(in)
Yes, this is a glorious Qur'an on a tablet well guarded.
It is thought of as containing the heavenly original of the Qur'an. Muslims in general believe that has 104 parts.
The Eternal Tablet:
104 Divisions
- 100 were later taken back, of these were
* 10 to Adam
* 50 to Seth
* 30 to Enoch
* 10 to Abraham
- 1 to Moses (Torah)
- 1 to David (Zaboor)
- 1 to Jesus (Injil)
- 1 to Muhammad : the Qur'an
Throughout the Qur'an revelation from God is thought to be pre-existing and "sent down" to mankind at various times, e.g.
According to the understanding of some Muslims, the Torah, Zaboor, and Injil are still left here (i.e. not taken back to heaven) because their purpose is to predict the coming of Muhammad (cf. as-Saff 61:6; al-A`raf 7:157).
The claim of another uncreated, eternal entity besides God, is a mystery (or a problem?) in Muslim theology. See this sample dialog for an illustration of the argument. The article God, His Word, and the Qur'an in Islam gives a very thorough discussion of it.