Reference: Fataawa li Ahlil-Mayyit – Question 29, Page 32
al-Muntaqaa min Fataawa al-Fowzaan – Volume 1, Page 194
Question: Some people place a sign with a big stone made of marble or some other specific sign to indicate the grave of the deceased so as to facilitate future visits without doubt as to the location of the grave.
Response: There is no harm in placing a large stone as an indicator by which the grave can be identified during [future] visits. This is because the Messenger (sal-Allaahu ‘alayhi wa sallam) did this with the grave of ‘Uthmaan.
 Obviously not ‘Uthmaan ibn ‘Affaan – the third khaleefah [after Abu Bakr and ‘Umar), who died after the Messenger (sal-Allaahu ‘alayhi wa sallam) himself.