It is the belief (aqīdah) of theAhle-Sunnah-wal-Jamā’hthat the punishment in the grave is a reality which is proven from the Quran and ahādith. Hereunder are some references.

Verses from the Qurān:
النَّارُ يُعْرَضُونَ عَلَيْهَا غُدُوًّا وَعَشِيًّا.وَيَوْمَ تَقُومُ السَّاعَةُ أَدْخِلُوا آلَ فِرْعَوْنَ أَشَدَّ الْعَذَابِ
1.“And they will be exposed to the fire day and night. Then, on the day the hour will rise (day of judgement), it shall be said to the Angels: Cast the people of the Pharaoh into the severest punishment!”
(40:46)

Commentary:

·“Our scholars have substantiated the punishment in the grave with this verse of the Holy Qur’an. They say: ‘The verse indicates that fire will be brought to them morning and evening and this does not refer to the day of judgment as Allah says:“And on the day judgment will be established, cast you the people of Pharaoh in the severest penalty”(i.e. it is referring to a time that is before the day of judgment, m). It also does not refer to this world, as bringing them to the fire did not occur in this world. Therefore, it becomes evident that this happened after their death and will happen before the Day of Judgment, thus it proves the establishment of the punishment in the grave. When it is established and proved for Pharaoh’s people it will also be established for others, as there is no reason of distinction” [1]

·“This verse is a fundamental evidence for theAhle-Sunnah-wal-Jamā’hin establishing the punishment of the grave”. [2]
سَنُعَذِّبُهُم مَّرَّتَيْنِ ثُمَّ يُرَدُّونَ إِلَىٰ عَذَابٍ عَظِيمٍ
2.“We shall punish them twice and then they will be returned to a great punishment”
(9:101)

Commentary:

·Tabarani narrates from Sayyiduna Ibn Abbas (Allah be pleased with him) that he said:“Once on Friday the Messenger of Allah (Allah bless him & give him peace) gave a sermon.And then he said:“Allah has disgraced the hypocrites in the world and that is the first punishment and the second punishment will be in the grave” [3]
وَلَوْ تَرَىٰ إِذِ الظَّالِمُونَ فِي غَمَرَاتِ الْمَوْتِ وَالْمَلَائِكَةُ بَاسِطُو أَيْدِيهِمْ أَخْرِجُوا أَنفُسَكُمُ.الْيَوْمَ تُجْزَوْنَ عَذَابَ الْهُونِ بِمَا كُنتُمْ تَقُولُونَ عَلَى اللَّهِ غَيْرَ الْحَقِّ وَكُنتُمْ عَنْ آيَاتِهِ تَسْتَكْبِرُونَ

3.“If only you could see when the wrongdoers are in the throes of death and the Angels are stretching out their hands saying “Bring forth your souls! This day you shall be recompensed with the torment of disgrace because of what you used to say about Allah other than the truth. And you grew arrogant against his signs!”
(6:93)

Commentary:

·This is an address to the wrong doers while they are dying, informing them that they will be severely tormented on the very day of their death. If the punishment were to be postponed to the Day of Judgment, they would not have been told: “This dayyou shall be recompensed”. This proves that there is a torment in the grave.
References from theAhādīth:
عَنْ عَائِشَةَ قَالَتْ عَنِ النَّبِيِّ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ قَالَ: إِنَّ لِلْقَبْرِ ضَغْطَةً، وَلَوْ نَجَا أَوْ سَلِمَ أَحَدٌ مِنْهَا لَنَجَا سَعْدُ بْنُ مُعَاذٍ [4]
1.Hazrat Aisha radhiallahu anha narrates that Rasulullah sallallahu alaihi wasallam said: “Verily there is a constriction in the grave and if anyone was to pass it or be safe from it, it would be Sa’d bin Muādh.”
قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: »إِنَّمَا القَبْرُ رَوْضَةٌ مِنْ رِيَاضِ الجَنَّةِ أَوْ حُفْرَةٌ مِنْ حُفَرِ النَّارِ [5]
2.Rasulullahsallallahu alaihi wasallam said: “The grave is a garden from the gardens of Paradise or a pit from the pits of Hell”
عَنْ هَانِئٍ، مَوْلَى عُثْمَانَ، قَالَ: كَانَ عُثْمَانُ بْنُ عَفَّانَ إِذَا وَقَفَ عَلَى قَبْرٍ يَبْكِي حَتَّى يَبُلَّ لِحْيَتَهُ، فَقِيلَ لَهُ: تَذْكُرُ الْجَنَّةَ وَالنَّارَ، وَلَا تَبْكِي، وَتَبْكِي مِنْ هَذَا؟ قَالَ: إِنَّ رَسُولَ اللَّهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ، قَالَ: إِنَّ الْقَبْرَ أَوَّلُ مَنَازِلِ الْآخِرَةِ، فَإِنْ نَجَا مِنْهُ، فَمَا بَعْدَهُ أَيْسَرُ مِنْهُ، وَإِنْ لَمْ يَنْجُ مِنْهُ، فَمَا بَعْدَهُ أَشَدُّ مِنْهُ
قَالَ: وَقَالَ رَسُولُ اللَّهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ: »مَا رَأَيْتُ مَنْظَرًا قَطُّ إِلَّا وَالْقَبْرُ أَفْظَعُ مِنْهُ [6]
3.Hāni’ the freed slave of Hadhrat Utmānradhiallahu anhunarrates that when Hadhrat Uthmān ibn Affān would stand in front of a grave, he would cry until his beard would become moist. It was once said to him. You mention Heaven and Hell and you don’t cry yet you cry from this? He replied “Rasulullahsallallahu alaihi wasallamsaid “Verily the grave is the first hurdle from the hurdles of the Hereafter. If one succeeds in it, then what is to come after it will be easier for him. However, if does not succeed in it, then what is to come after it will be harder for him.”
He also said that Rasulullahsallallahu alaihi wasallamsaid: “I have not seen a sight except that the grave is more horrific than it.”............
http://www.askimam.org/public/question_detail/19504