How the Divine Revelation used to be revealed and what was the first thing revealed Sahih al-Bukhari 4978, 4979
Arabic hadith text
حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ مُوسَى، عَنْ شَيْبَانَ، عَنْ يَحْيَى، عَنْ أَبِي سَلَمَةَ، قَالَ أَخْبَرَتْنِي عَائِشَةُ، وَابْنُ، عَبَّاسٍ رضى الله عنهم قَالاَلَبِثَ النَّبِيُّ صلى الله عليه وسلم بِمَكَّةَ عَشْرَ سِنِينَ يُنْزَلُ عَلَيْهِ الْقُرْآنُ وَبِالْمَدِينَةِ عَشْرَ سِنِينَ.
Translation
Narrated `Aisha and Ibn `Abbas
The Prophet (ﷺ) remained in Mecca for ten years, during which the Qur'an used to be revealed to him; and he stayed in Medina for ten years.