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

The purpose of burial is to hide the body in a pit in order to prevent its stench from fouling the atmosphere, and to save it from being eaten by various beasts and birds. If these conditions are met and this purpose is served, then one has carried out one’s responsibility. It is encouraged, however, to make the depth of the grave equal to the height of an average man. This is based on a hadith reported by Nasa’i and Tirmizhi (who considers it a sound hadith) from Hisham ibn ‘Amer who said: “We complained to the Prophet, peace be upon him, on the day of the battle of Uhud, saying: ‘O Allah’s Messenger! Digging a separate grave for every body is a very hard job.’ The Prophet, peace be upon him, said: ‘Dig, dig deeper, dig well, and bury two or three bodies in each grave.’ The Companions asked him: ‘Who should be put in the graves first?’ The Prophet, peace be upon him, said: ‘Put those most learned in the Qur’an first.’ My father, the narrator added, was the third of the three who were put in one grave.”

Ibn Abi Shaibah and Ibn Al-Munzhir reported that ‘Umar said: “Dig a grave deep enough for a man’s height and make it broader.” According to Abu Hanifah and Ahmad the grave should be deep enough for half of the average height of a man, but if dug deeper, it is better.