How the Divine Revelation used to be revealed and what was the first thing revealed
حَدَّثَنَا مُوسَى بْنُ إِسْمَاعِيلَ، حَدَّثَنَا مُعْتَمِرٌ، قَالَ سَمِعْتُ أَبِي، عَنْ أَبِي عُثْمَانَ، قَالَ أُنْبِئْتُ أَنَّ جِبْرِيلَ، أَتَى النَّبِيَّ صلى الله عليه وسلم وَعِنْدَهُ أُمُّ سَلَمَةَ فَجَعَلَ يَتَحَدَّثُ فَقَالَ النَّبِيُّ صلى الله عليه وسلم لأُمِّ سَلَمَةَ " مَنْ هَذَا ". أَوْ كَمَا قَالَ قَالَتْ هَذَا دِحْيَةُ. فَلَمَّا قَامَ قَالَتْ وَاللَّهِ مَا حَسِبْتُهُ إِلاَّ إِيَّاهُ حَتَّى سَمِعْتُ خُطْبَةَ النَّبِيِّ صلى الله عليه وسلم يُخْبِرُ خَبَرَ جِبْرِيلَ أَوْ كَمَا قَالَ، قَالَ أَبِي قُلْتُ لأَبِي عُثْمَانَ مِمَّنْ سَمِعْتَ هَذَا. قَالَ مِنْ أُسَامَةَ بْنِ زَيْدٍ.
Narrated Abu `Uthman: I was informed that Gabriel came to the Prophet (ﷺ) while Um Salama was with him. Gabriel started talking (to the Prophet). Then the Prophet (ﷺ) asked Um Salama, "Who is this?" She replied, "He is Dihya (al-Kalbi)." When Gabriel had left, Um Salama said, "By Allah, I did not take him for anybody other than him (i.e. Dihya) till I heard the sermon of the Prophet (ﷺ) wherein he informed about the news of Gabriel." The subnarrator asked Abu `Uthman: From whom have you heard that? Abu `Uthman said: From Usama bin Zaid.