Hadithcore
Sahih Muslim, 160 b
sahih

This hadith has been narrated on the authority of 'A'isha with another chain of narrators like one transmitted by Yunus, i. e. the first thing with which the revelation was initiated with the Messenger of Allah (ﷺ) except the words:

By Allah, Allah would never humiliate you, and Khadija said: O son of my uncle! Listen to the son of your brother.

وَحَدَّثَنِي مُحَمَّدُ بْنُ رَافِعٍ، حَدَّثَنَا عَبْدُ الرَّزَّاقِ، أَخْبَرَنَا مَعْمَرٌ، قَالَ قَالَ الزُّهْرِيُّ وَأَخْبَرَنِي عُرْوَةُ، عَنْ عَائِشَةَ، أَنَّهَا قَالَتْ أَوَّلُ مَا بُدِئَ بِهِ رَسُولُ اللَّهِ صلى الله عليه وسلم مِنَ الْوَحْىِ ‏.‏ وَسَاقَ الْحَدِيثَ بِمِثْلِ حَدِيثِ يُونُسَ ‏.‏ غَيْرَ أَنَّهُ قَالَ فَوَاللَّهِ لاَ يُحْزِنُكَ اللَّهُ أَبَدًا ‏.‏ وَقَالَ قَالَتْ خَدِيجَةُ أَىِ ابْنَ عَمِّ اسْمَعْ مِنِ ابْنِ أَخِيكَ ‏.‏

References2 variants
In-Book Reference
Book 1, Hadith 309
USC-MSA web (English) reference
Book 1, Hadith 302 (deprecated numbering scheme)