Is it true that prophets are alive in their graves? What is the proof?
Ahl as-Sunnah holds that the life of prophets in their graves is firmly established through authentic and explicit proofs. The great hadith master, Imam Bayhaqi compiled them in a separate book. Imam Jalal Al-Suyuti, M.H. , also authored a book on this topic. May Allah Most High have mercy on them both. Ibn Hajar , M.H., says:
Al-Bayhaqi compiled a brilliant book regarding the life of prophets in their graves in which he mentions the hadith of Anas, "The prophets are alive in their graves praying", which he  transmits from the channel of Yahya Ibn Abu Katheer, who is a rigorously authenticated transmitter, on the authority of Al-Mustalim Ibn Sa'eed, whom Ahmad and Ibn Hibban deem reliable, on the authority of Al-Hajjaj Al-Aswad (a.k.a. Ibn Abu Ziyad Al-Basri), whom Ahmad and Ibn Ma'een deem reliable, on the authority of Thabit, on the authority of him .
Abu Ya'la also transmits it in his Musnad from this avenue. Al-Bazzar transmits it, but it appears with him on the authority of Hajjaj Al-Sawwaf, which is an error. Al-Hajjaj Al-Aswad is the correct name, as is explicitly mentioned in the narration of Al-Bayhaqi, which he rigorously authenticates.
This hadith is an authentic, explicit text that establishes the life of prophets in their graves and that they pray in them. Imam Bayhaqi supports what is mentioned in this hadith with what has been established in Saheeh Muslim in terms of the narration of Hammad Ibn Salamah, on the authority of Anas, on the Prophet (Allah bless him and grant him peace), who said, "I passed by Moses at the red dune on the night in which I was transported  and he was standing in his grave praying." He  also narrates it from another avenue on the authority of Anas. Al-Bayhaqi says:
If it is argued that this is specific to Moses, we say that we find supporting evidence in the hadith of Abu Hurayra, also in Saheeh Muslim, from the channel of Abdullah Ibn Al-Fadhl, on the authority of Abu Salamah, on the authority of Abu Hurayra (who promotes  it), "I found myself in the Hijr  and Quraish was asking me about my night journey ... and I found myself amidst a group of prophets and Moses was standing in prayer ... and Jesus Son of Mary was standing in prayer - the one who resembles him most being 'Urwa Ibn Mas'ud - and Abraham was standing in prayer - the one who resembles him most being your companion  - then the prayer time entered and I led them in it."
In the hadith of Sa'eed Ibn Al-Musayyib, on the authority of Abu Hurayra, it mentions, "that he met them in Jerusalem and then the time of prayer entered, so our prophet (Allah bless him and grant him peace) led them and then they gathered in Jerusalem." In the hadith of Abu Dharr and Malik Ibn Sa'sa'ah regarding the story of the Night Journey, it mentions that he met them in the heavens.
These channels are rigorously authenticated, so they can be correlated as him having seen Moses standing in prayer in his grave, then he and the mentioned prophets ascended to the heavens, where he met them, then they gathered in Jerusalem. The time for prayer entered, and our prophet (Allah bless him and grant him peace) led them. [Al-Bayhaqi] says, "Their praying at different times and in different places is not logically refutable, and it has been established through transmission, so it indicates their life."
Ibn Hajar, M.H., transmits these words of Al-Bayhaqi and doesn't criticize them. Rather, he adds:
In my opinion, while their life is established through transmission, reason strengthens this, since martyrs are alive according to the text of the Qur'an, and prophets are better than martyrs.
Other supporting evidence for the hadith is found in what Abu Dawud transmits regarding the hadith of Abu Hurayra, which contains, "Pray for me, for surely, your prayers reach me wherever you are." It's chain of narrators is rigorously authenticated. Abu Al-Shaykh transmits it in Kitab Al-Thawab with a good chain of narrators with the wording, "Whoever prays for me at my grave, I hear him, and whoever prays for me remotely, it is imparted to me." Ibn Khuzayma and others have rigorously authenticated what is in Abu Dawud and Al-Nasa'i, on the authority Aws Ibn Aws (who promotes it), regarding the virtue of Friday, "'Increase in your prayers for me in it, for truly, your prayers are presented before me.' They said, 'Oh Messenger of Allah, how will our prayers be presented before you when you have decomposed?' He said, 'Allah has forbidden the earth to consume the bodies of prophets.'"
Answered by Shaykh Amjad Rasheed [Translated by Moustafa Elqabbany]
1. M.H., Master of Hadith (Ar. hafidh), one who has memorized 100,000 hadiths with their complete chain of narrators.
2. There are two major Shafi'i figures with the name Ibn Hajar: Ibn Hajar Al-Asqallani, author of Fath Al-Bari, an exegesis of Sahih Al-Bukhari, and Ibn Hajar Al-Haytami, author of Tuhfa Al-Muhtaj, an authoritative expounding of the Minhaj of Imam Nawawi. Ibn Hajar Al-Asqallani acquired the title of Commander of the Faithful in the Sciences of Hadith, and is without equal in his field, bar none. Ibn Hajar Al-Haytami, along with Al-Ramli, define the authoritative and final positions of the Shafi'i school in the Tuhfa and Nihaya, their respective exegeses of the Minhaj.
3. i.e. Al-Bayhaqi.
4. i.e. Anas (Allah be pleased with him).
5. i.e. The night of Isra' and Mi'raj.
6. i.e. Al-Bayhaqi.
7. Promoting (Ar. rafa`a) a narration means attributing it to the Prophet (Allah bless him and grant him peace).
8. i.e. The Hijr of Ka'ba.
9. Your companion: The Prophet (Allah bless him and give him peace) is referring to himself.