Mount Hira (Jabal Hira), which lies about two miles from the Ka’bah. Near the top is a small cave, a little less than 4 meters in length and a little more than one and a half meters in width. It was here that the Prophet Muhammad (ﷺ) received the first revelations of the Holy Quran during the month of Ramadhan in 610 CE. The mountain is also known as Jabal Noor (the Mountain of Light).
The Prophet (ﷺ) first began to have revelations in the form of good dreams which came true. Then he began to like solitude. He would go to the cave of Hira and meditate there in solitude for a number of days and nights. He would take provisions with him to stay for an extended period, and when he returned to Khadija (may Allah be pleased with him), he would stock up again and go back to the cave. This was his practice until Truth was revealed to him by an angel while he was in the cave of Hira.
During Tahajjud time one night, when he was alone in the cave, there came to him an angel in the form of a man. The angel said to him, “Recite!”. “I cannot read”, the Prophet (ﷺ) replied. The angel took hold of him a second time and pressed him until he could not endure it any longer. After letting him go, the angel again said, “Recite!”. Again the Prophet (ﷺ) replied “I cannot read”. The angel further embraced him again until he had reached the limit of endurance and said “Recite!” for the third time the Prophet (ﷺ) said “I cannot read”. The angel released him and said: “Read in the name of your Lord, the Creator. He Who created man from a clot. Read! And your Lord is the Most Bounteous. Who taught by the Pen, taught man what he knew not.” [96:1-5]
This was the first day of his Prophethood and these were the first verses of the Quran revealed to him. He recited these words after the angel, who thereupon left him; and he said; “It was as though the words were written on my heart.” He was very alarmed by the experience and feared that he had become possessed.
The Prophet (ﷺ) fled from the cave, and when he was halfway down the slope of the mountain he heard a voice above him saying: “O Muhammad, thou art the Messenger of God, and I am Jibraeel (Gabriel)”. The Prophet (ﷺ) stood gazing at the angel; then he turned away from him, but whichever way he looked the angel was always there on the horizon, whether it was to the north, east, south or west. Finally the angel turned away. The Prophet (ﷺ) was terrified and his heart was pounding hard. He returned home to Khadija (may Allah be pleased with her) and said, “Cover me!”. Khadija (may Allah be pleased with her) covered him and helped him calm down.
The Quran tells us that the first revelation descended in Ramadan on the ‘Night of Power’.“The month of Ramadan is the month in which the Quran was revealed.” [2:185]
“We have indeed revealed this message during the Night of Power.” [97:1]
The entire Quran was revealed over a period of approximately 22 years, 5 months and 14 days
Source: Islamic Landmarks