Is it permissible to mark a grave with a stone or piece of wood?

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It is permissible to mark a grave with a stone or wood. Imam Ahmad permitted a man to mark the grave with a distinguishing sign. The Prophet, peace and blessings be upon him, marked the grave of Uthman ibn Maz'un. Evidence for this is the narration where the Prophet commanded a man to bring a stone for the grave of Uthman ibn Maz'un. When the man could not lift it, the Messenger of Allah rolled up his sleeves, carried it, and placed it at the head of the grave, stating, 'I mark with it the grave of my brother, and those of his family who die shall be buried near him.' This narration is transmitted via Abu Dawud and Ibn Majah from the narration of Anas.