1. To have Faith (Eemaan) in the punishment of the grave has the consensus (ijmaa) of Ahlus-Sunnah wal-Jamaaah, as has been declared by Abul-Hasan al-Asharee (Risaalah ilaa Ahlith-Thaghr, p.279), being denied only by the Khawaarij and some of the Mutazilah.
Allah, the Most High, says:
In front of the Fire will they be brought, morning and evening, and on the Day when the Hour will be established (it will be said to the angels), Cause Pharaohs people to enter the severest torment! Soorah Ghaafir (40): 46
Ibn Abbas said, Allahs Messenger passed by two graves and said, Indeed, they are being punished, but they are not being punished for something major. Then he said, Rather it is so: one of them did not protect himself from being soiled with his urine; the other used to go about telling stories (to cause enmity).... Reported by al-Bukhaaree (Eng. trans. 1/141/no.215), Ahmad and the compilers of the four Sunan.
Abu Hurairah reports that Allahs Messenger said, When the deceased (or one of you) is put in the grave, two black angels having blue eyes come to him. One of them is called al-Munkar and the other an-Nakeer and they will say..... Reported by at-Tirmidhee and declared hasan by Shaikh al-Albaanee in Saheeh al-Jaami.
There are many ahaadeeth concerning the punishment of the grave. Al-Baihaqee wrote a whole treatise entitled Ithbaat Adhaab al-Qabr, consisting of some 240 narrations.
Imam ash-Shaafiee (d.204H) said, The punishment of the grave is true, the questioning of those in the graves is true, the Resurrection is true, the Day of Judgement is true, Paradise and the Fire are true. Whatever else is reported in the Sunnah and so mentioned by the scholars and their followers throughout the lands of the Muslims is true. Reported by al-Baihaqee in Manaaqib ash-Shaafiee (1/415).
Imaam Ahmad ibn Hanbal (d.241H) said, The principles of the Sunnah according to us are: to cling to that which the Companions of Allahs Messenger were upon and to follow them...to have Faith in the punishment of the grave and that this Ummah will be tested in their graves and asked about Faith and Islaam, who is his Lord and who is his Prophet. Munkar and Nakeer will come to him as Allah wills and however Allah wishes. From Usool us-Sunnah of Imaam Ahmad.
Imaam Ahmad said, Punishment of the grave is true. No-one denies it except one who is misguided and astray and leading others astray. Reported by Ibn Abee Yalaa in Tabaqaatul-Hanaabilah (1/174).