The revelation of the Qur'an began in the laila al-qadr of Ramadan (the 27th night or one of the odd nights after the 21st) after the Prophet Muhammad had passed the fortieth year of his life (that is around the year 610), during his seclusion in the cave of Hira' on a mountain near Makka.

[English translations of ahadith are, unless otherwise indicated, from Khan, Muhammad Muhsin: The Translation of the Meanings of Sahih al-Bukhari, 9 vols., Istanbul, 1978 (abbr. as Bukhari) and Siddiqui, Abdul Hamid: Sahih Muslim, 4 vols., Lahore, 1978 (abbr. as Muslim).] This is the account, as reported in the Sahih of Bukhari:

Narrated Aisha the mother of the faithful believers: The commencement of the divine inspiration to Allah's apostle was in the form of good dreams which came like bright daylight (i.e. true) and then the love of seclusion was bestowed upon him.

He used to go in seclusion in the Cave of Hira', where he used to worship (Allah alone) continuously for many days before his desire to see his family. He used to take with him food for the stay and then come back to (his wife) Khadija to take his food likewise again, till suddenly the truth descended upon him while he was in the Cave of Hira'.

The angel came to him and asked him to read. The Prophet replied 'I do not know how to read'. The Prophet added, 'The angel caught me (forcibly) and pressed me so hard that I could not bear it any more. He then released me and again asked me to read and I replied, "I do not know how to read".