When Uthman ibn Maz'un died, he was brought out on his bier and buried. The Prophet (sallallaahu alayhi wa sallam) ordered a man to bring him a stone, but he was unable to carry it. The Messenger of Allah (sallallaahu alayhi wa sallam) got up and going over to it rolled up his sleeves.
The narrator Kathir told that al-Muttalib remarked: The one who told me about the Messenger of Allah (sallallaahu alayhi wa sallam) said: I still seem to see the whiteness of the forearms of the Messenger of Allah (sallallaahu alayhi wa sallam) when he rolled up his sleeves. He then carried it and placed it at his head saying: I am marking my brother's grave with it, and I shall bury beside him those of my family who die.
[Sunan Abi Dawud Hadith No. 3206]
Based on this Hadith Imaam Ahmad bin Hanbal said:
قال أحمد لا بأس أن يعلم الرجل القبر علامة يعرفه بها
There is no harm if a person marks the grave in order to recognize who the grave belongs to.
The hadiith has been graded hasan by Sheikh al-'albaani.
al-Mughni Volume 2 Page 376