Ayesha reported,

The first form of revelation that came to the Messenger of Allah ﷺ was true dreams during sleep. He would not see a dream except that it came to pass as clearly as the break of dawn. Then solitude was made dear to him, and he would withdraw to the cave of Hira, where he engaged in worship for many nights before returning to his family. He would take provisions for that purpose, then return to Khadijah and take more provisions for a similar period, until the truth came to him while he was in the cave of Hira.The angel came to him and said, "Recite."He replied, "I am not one who recites."The Prophet said, "He took hold of me and pressed me until I could bear no more, then released me and said, 'Recite.'"I said, "I am not one who recites."So he took hold of me and pressed me a second time until I could bear no more, then released me and said, "Recite."I said, "I am not one who recites."So he took hold of me and pressed me a third time, then released me and said, "Recite in the name of your Lord who created. Created man from a clinging clot. Recite, and your Lord is the Most Generous. Who taught by the pen. Taught man that which he knew not." [Alaq 96:1–5].The hadith continues at length.

عن عائشة أنها قالت

أول ما بدئ به رسول الله ﷺ من الوحي الرؤيا الصالحة في النوم، فكان لا يرى رؤيا إلا جاءت مثل فلق الصبح ثم حبب إليه الخلاء وكان يخلو بغار حراء فيتحنث فيه- وهو التعبد- الليالي ذوات العدد قبل أن ينزع إلى أهله، ويتزود لذلك، ثم يرجع إلى خديجة فيتزود لمثلها، حتى جاءه الحق وهو في غار حراء فجاءه الملك فقال: اقرأ قال: ما أنا بقارئ قال: فأخذني فغطني حتى بلغ مني الجهد ثم أرسلني، فقال: اقرأ قلت: ما أنا بقارئ فأخذني فغطني الثانية حتى بلغ مني الجهد ثم أرسلني فقال: اقرأ فقلت: ما أنا بقارئ فأخذني فغطني الثالثة ثم أرسلني فقال: {اقرأ باسم ربك الذي خلق خلق الإنسان من علق اقرأ وربك الأكرم الذي علم بالقلم علم الإنسان ما لم يعلم} [سورة العلق: ١- ٥]. الحديث بطوله.