How was the Quran revealed to Prophet Muhammad?

Mostly through the angel Jibril (Gabriel), in parts, over about 23 years. Muslims believe Allah sent the Quran down to the Prophet ﷺ not all at once but gradually, in response to events and questions. Jibril delivered the words, the Prophet ﷺ memorised them, and the Companions memorised and wrote them down.

Through the angel Jibril

The Quran describes the messenger of revelation as "the trustworthy spirit" who brought it down "into your heart" so that the Prophet ﷺ would be a warner, "in a clear Arabic tongue" (Quran 26:192-195). The Prophet ﷺ did not compose the words. He received them and recited them as he heard them. See Who is Jibril in Islam?

What it was like

A Companion named al-Harith ibn Hisham asked the Prophet ﷺ how revelation came to him. He replied: "Sometimes it is revealed like the ringing of a bell, this form of inspiration is the hardest of all, and then this state passes off after I have grasped what is inspired. Sometimes the Angel comes in the form of a man and talks to me and I grasp whatever he says." Aisha, who reported this, added that she saw him receiving revelation on a very cold day, and sweat dropped from his forehead when it was over (Sahih al-Bukhari 2).

In stages, not all at once

The Quran explains why it came gradually:

وَقُرْءَانًا فَرَقْنَـٰهُ لِتَقْرَأَهُۥ عَلَى ٱلنَّاسِ عَلَىٰ مُكْثٍ وَنَزَّلْنَـٰهُ تَنزِيلًا "[It is] a Quran We have revealed in stages so that you may recite it to people at a deliberate pace. And We have sent it down in successive revelations." Quran 17:106

This gave the Prophet ﷺ and his community time to understand, memorise and live by each part. It also allowed revelation to answer real events, such as questions from the people, hardships and new situations. The period lasted about 23 years, roughly 13 in Makkah and 10 in Madinah, as is commonly stated. For why the Quran is not in the order of revelation, see Why isn't the Quran in chronological order?

Where and when it began

Revelation began in the month of Ramadan. The Quran says: "Ramadan is the month in which the Quran was revealed as a guide for humanity" (Quran 2:185). The Prophet ﷺ used to spend time in worship in the cave of Hira near Makkah, and there the angel came to him and told him to read. He answered that he could not read, and the angel pressed him three times before saying, "Read in the name of your Lord, who has created (all that exists)" (Sahih al-Bukhari 3). See What was the first verse of the Quran revealed?

Reviewed every year

Abu Hurairah reported that Jibril used to review the recitation of the Quran with the Prophet ﷺ once a year, and twice in the year he died (Sahih al-Bukhari 4998). This regular review, along with memorisation and writing, helped the Quran stay exact.

Key points

The Quran was brought by the angel Jibril to the Prophet's ﷺ heart (Quran 26:192-195).
Revelation came in different ways, sometimes like a bell (Sahih al-Bukhari 2).
It was revealed in stages over about 23 years (Quran 17:106).
Jibril reviewed it with the Prophet ﷺ every year (Sahih al-Bukhari 4998).

Tip: To feel the scale of this, read Surah al-Alaq (96) and then Surah al-Baqarah (2) in order. The first is revealed in Makkah, the second in Madinah, and you can sense the change in voice.