Short answer
In the cave of Hira, Jibril said "Read!" (Iqra). The Prophet ﷺ replied "I am not a reader." Jibril squeezed him tightly three times and then recited the first verses of Surah al-Alaq: "Read in the name of your Lord who created."
The Sunni account, from Aisha in Sahih al-Bukhari and the seerah books, is that the first revelation came in the cave of Hira, in Ramadan when the Prophet ﷺ was 40, and that the first verses revealed were 96:1-5.
The four schools agree on this.
Explanation
Before revelation, the Prophet ﷺ began to love solitude. He would stay in the cave of Hira near Makkah for nights of worship. He also saw true dreams that came true like the light of dawn. Then, in Ramadan, when he was 40, around 610 CE, the angel came to him.
Jibril said "Read!" He said "I am not a reader." The angel pressed him hard until he felt the strain, released him and said "Read!" again. This happened three times. Then Jibril recited: "Read in the name of your Lord who created, created man from a clinging substance. Read, and your Lord is the Most Generous, who taught by the pen, taught man what he did not know."
Why the squeeze? Scholars such as Ibn Hajar mention several wisdoms: to turn his whole attention to what was coming, to show him this was real and not a dream, and to prepare him for the weight of revelation, which would be heavy (73:5).
He came home trembling and said to Khadijah: "Cover me, cover me." When the fear passed he told her what happened. She replied: "Never! By Allah, Allah will never disgrace you," and listed his kindness to relatives, the weak and guests. She took him to her cousin Waraqa ibn Nawfal, a Christian scholar, who said this was the same angel that came to Musa. Read more on Prophet Muhammad ﷺ and Khadijah.
The evidence
Quran, Surah Al-Alaq 96:1-3
بِسْمِ ٱللَّهِ ٱلرَّحْمَٰنِ ٱلرَّحِيمِ ٱقْرَأْ بِٱسْمِ رَبِّكَ ٱلَّذِى خَلَقَ ﴿١﴾ خَلَقَ ٱلْإِنسَٰنَ مِنْ عَلَقٍ ﴿٢﴾ ٱقْرَأْ وَرَبُّكَ ٱلْأَكْرَمُ ﴿٣﴾
Recite in the name of your Lord who created - Created man from a clinging substance. Recite, and your Lord is the most Generous -
The first words revealed.
Quran, Surah Al-Muzzammil 73:5
إِنَّا سَنُلْقِى عَلَيْكَ قَوْلًۭا ثَقِيلًا ﴿٥﴾
Indeed, We will cast upon you a heavy word.
Revelation is weighty, one reason given for the squeeze.
Sahih al-Bukhari 3
Sahih... فغطني حتى بلغ مني الجهد ...
"He pressed me until I felt the strain."
The squeeze in the cave of Hira.
Sahih al-Bukhari 3
Sahih... كلا والله ما يخزيك الله أبدا ...
"Never! By Allah, Allah will never disgrace you."
Khadijah's comfort after the first revelation.
Where the ruling is recorded
- Fath al-Bari (Ibn Hajar), Commentary on Kitab Bad' al-Wahy, the hadith of Aisha on how revelation began. Explains the squeeze and the meaning of 'I am not a reader'.
- Sirat Ibn Hisham, Account of the beginning of revelation. Places it in Ramadan in the cave of Hira.
- al-Rahiq al-Makhtum (al-Mubarakpuri), Chapter on the cave of Hira and the coming of Jibril. A clear modern summary.
This answer is pending review by the scholars on our team. 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.