Fiqh-us-Sunnah Volume 4, Funerals and Dhikr, Fiqh 4.062B.

Lahd is a crevice on the side of a grave facing the qiblah, which is covered with unburnt bricks like a house with a roof. A regular grave, on the other hand, is a pit dug in the ground, with the body placed in it and then sealed off with un-burnt bricks and covered to form a ceiling. Either of these two methods is permissible, but the first one land is preferable in the light of a hadith reported by Ahmad and Ibn Majah on the authority of Anas who said: “When the Prophet, peace be upon him, died, there were two grave diggers. One usually dug the land and the other a regular tomb-like grave. The Companions said: ‘Let us seek guidance from our Lord.’ Then they asked each of them to dig a grave, and decided the grave of the one who finished first be chosen for the burial of the Prophet’s remains. The one who dug the land finished first, so they buried the remains of the Prophet, peace be upon him, in a land.”

This hadith shows that both forms are permissible. The fact that the land is preferable is indicated by a tradition transmitted by Ahmad and the Compilers of the Sunan on the authority of Ibn ‘Abbas who reported: “The Prophet, peace be upon him, said: ‘Land is for us, and digging a pit (i.e., a regular grave) for others.”‘