Prohibition of writing anything on a gravestone
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Question: Is it permissible to write the name of the deceased or an aayah from the Qur.aan on a headstone by the grave ?

Response: It is not permissible to write the name of the deceased on a headstone or on anything else by the grave, since the Messenger (sal-Allaahu `alayhe wa sallam) forbade that, even if it be an aayah from the Qur.aan, or a single word, or even a single letter - this is (all) impermissible.

However, if the grave is indicated with a sign other than writing, so as to identify it for (future) visits and extending the greeting (upon visiting), such as defining a line (or groove in the earth), or placing a stone by the grave without any writing on it, then there is no harm in this.

As for writing, then this is not permissible, because the writing is a means to shirk. So (sometime later) a generation may come where the people will say: "Indeed the details of this person have only been written on the gravestone because this person was good and benefitted the people." And as such, worshipping the grave has occurred.

Shaykh Ibn Fowzaan
Fataawa li Ahlil-Mayyit - Question 30, Page 32
al-Muntaqaa min Fataawa al-Fowzaan - Volume 1, Page 195