Sahih al-Bukhari 3896
Narrated Hisham's father:
Khadija died three years before the Prophet (ﷺ) departed to Medina. He stayed there for two years or so 
and then he married `Aisha when she was a girl of six years of age, and he consumed that marriage 
when she was nine years old.
حَدَّثَنِي عُبَيْدُ بْنُ إِسْمَاعِيلَ، حَدَّثَنَا أَبُو أُسَامَةَ، عَنْ هِشَامٍ، عَنْ أَبِيهِ، قَالَ تُوُفِّيَتْ خَدِيجَةُ قَبْلَ مَخْرَجِ النَّبِيِّ صلى الله عليه وسلم إِلَى الْمَدِينَةِ بِثَلاَثِ سِنِينَ، فَلَبِثَ سَنَتَيْنِ أَوْ قَرِيبًا مِنْ ذَلِكَ، وَنَكَحَ عَائِشَةَ وَهْىَ بِنْتُ سِتِّ سِنِينَ، ثُمَّ بَنَى بِهَا وَهْىَ بِنْتُ تِسْعِ سِنِينَ‏.‏