Fiqh-us-Sunnah Volume 4, Funerals and Dhikr, Fiqh 4.068A.

It is permissible to place a mark, such as a stone or a piece of wood, over a grave so that it can be recognized. This is based on a hadith reported by Ibn Majah from Anas that the Prophet, peace be upon him, “placed a rock over ‘Uthman ibn Maz’eun’s grave so that it could be recognized.” In az-Zawa’id it is stated that its chain of narrators is sound. Abu Daw’ud reported it in the hadith of al-Muttalib ibn Abi Wada’ah which has the following words: “He carried a rock and placed it by the head-side of the grave and said, ‘I want to mark my brother’s grave and then bury beside it whoever else of my family dies.”‘ This hadith indicates that it is preferable for relatives of the deceased to be buried in adjacent spots because it makes it easier for those who visit their graves to pray for them.