Short answer
Wahy (revelation) is Allah's communication of His message to a prophet. It came in different ways: through the angel Jibril, in true dreams, by inspiration placed in the heart, and by Allah speaking from behind a veil. The Quran is revelation in wording and meaning; the Sunnah is revelation in meaning.
Belief that Allah sent revelation to His prophets is part of iman. Sunni scholars hold that prophetic revelation ended with the Prophet Muhammad ﷺ, so no later inspiration, dream or claim can add to the religion or change the Shariah.
The four schools agree on this.
Explanation
The Quran names the ways Allah speaks to a human being: "by revelation or from behind a partition or that He sends a messenger to reveal, by His permission, what He wills" (42:51). Revelation to the prophets usually came through the angel Jibril, whom the Quran calls the Trustworthy Spirit (26:193). Allah spoke to Musa from behind a veil, and to the Prophet ﷺ on the night of the Mi'raj.
The Prophet ﷺ described how it came to him. Sometimes it was like the ringing of a bell, which was the hardest on him; sometimes the angel came in the form of a man and spoke to him. Revelation began with true dreams, then Jibril came to him in the cave of Hira with the first words of Surah al-Alaq. The Quran came down in parts over about twenty-three years.
Scholars divide revelation into two kinds. The Quran is recited revelation (wahy matluw): its words and meaning are from Allah, and it is recited in prayer. The Sunnah is revelation in meaning, expressed in the Prophet's own words, which is why the Quran says he "does not speak from his own inclination" (53:3-4) in matters of religion.
The Quran also uses the word wahy in a wider sense, for Allah's inspiration to the bee and to the mother of Musa. That is not prophetic revelation. Read What is the Quran? and the history of the Quran.
The evidence
Quran, Surah Ash-Shura 42:51
۞ وَمَا كَانَ لِبَشَرٍ أَن يُكَلِّمَهُ ٱللَّهُ إِلَّا وَحْيًا أَوْ مِن وَرَآئِ حِجَابٍ أَوْ يُرْسِلَ رَسُولًۭا فَيُوحِىَ بِإِذْنِهِۦ مَا يَشَآءُ ۚ إِنَّهُۥ عَلِىٌّ حَكِيمٌۭ ﴿٥١﴾
And it is not for any human being that Allah should speak to him except by revelation or from behind a partition or that He sends a messenger to reveal, by His permission, what He wills. Indeed, He is Most High and Wise.
Lists the ways Allah communicates with His messengers.
Quran, Surah An-Najm 53:4
إِنْ هُوَ إِلَّا وَحْىٌۭ يُوحَىٰ ﴿٤﴾
It is not but a revelation revealed,
Shows the Prophet's teaching in religion comes from revelation.
Sahih al-Bukhari 2
Sahih... أَحْيَانًا يَأْتِينِي مِثْلَ صَلْصَلَةِ الْجَرَسِ وَهُوَ أَشَدُّهُ عَلَىَّ ...
"Sometimes it comes to me like the ringing of a bell, and that is the hardest on me."
Describes how revelation came to the Prophet ﷺ.
Where the ruling is recorded
- al-Aqida al-Tahawiyya, Section on the Quran as the speech of Allah. States that the Quran is Allah's speech, sent down to His Prophet as revelation.
- al-Aqa'id al-Nasafiyya with Sharh al-Taftazani, Section on the sending of messengers. Explains that Allah sent messengers with revelation and that the last of them is Muhammad ﷺ.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.