What is the meaning of "the Qur'an was written on Israfil's forehead" after he was the first to prostrate to Adam?

What the questioner mentioned about the Qur'an being written on the forehead of Israfil has not been authentically narrated from the Prophet (peace be upon him) or any of the Companions. Rather, it has been narrated from some of the Tabi'in, but only what was narrated from Damra ibn Rabi'ah is sound, and this cannot be used as proof in matters of the unseen, which can only be known through revelation. There is no such narration from the Prophet (peace be upon him).

There are fabricated or very weak marfu' (attributed to the Prophet) hadiths, and mawquf (attributed to a Companion) narrations that are not sound, as well as some Isra'iliyat from Ka'b al-Ahbar and Wahb ibn Munabbih, but these are not established or reliable.

The correct belief is that the Noble Qur'an is written in the Preserved Tablet, as stated in His Almighty saying: "Rather, it is a glorious Qur'an, in a Preserved Tablet." [Al-Buruj: 21-22].