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

Jurists agree that it is permissible either to make a little hump over the grave or to flatten it. At-Tabari said: “I dislike it for a grave to be prepared in any way other than these two: it should be either levelled with the ground or raised with a hump over it, but no higher than one hand, as is the common practice among Muslims. Levelling the grave differs from flattening it completely. Jurists differ concerning which of these two methods is best. Al-Qadi ‘Iyad has reported from the major scholars that it is best to make a hump over the grave because Sufyan an-Nammar told him that he had seen the grave of the Prophet, peace be upon him, with a hump over it.” (Bukhari) This is the opinion of Abu Hanifah, Malik, Ahmad, al-Mazani, and many scholars of the Shafi’i school. Ash-Shafi’i’s opinion, however, is that because of the order of the Prophet, peace be upon him, levelling is best.