The Book of Marriage
وَحَدَّثَنَا عَبْدُ بْنُ حُمَيْدٍ، أَخْبَرَنَا عَبْدُ الرَّزَّاقِ، أَخْبَرَنَا مَعْمَرٌ، عَنِ الزُّهْرِيِّ، عَنْ عُرْوَةَ، عَنْ عَائِشَةَ، أَنَّ النَّبِيَّ صلى الله عليه وسلم تَزَوَّجَهَا وَهْىَ بِنْتُ سَبْعِ سِنِينَ وَزُفَّتْ إِلَيْهِ وَهِيَ بِنْتُ تِسْعِ سِنِينَ وَلُعَبُهَا مَعَهَا وَمَاتَ عَنْهَا وَهِيَ بِنْتُ ثَمَانَ عَشْرَةَ .A'isha (Allah be pleased with her) reported that Allah's Apostle (ﷺ) married her when she was seven years old, and he was taken to his house as a bride when she was nine, and her dolls were with her; and when he (the Holy Prophet) died she was eighteen years old
From Sahih Muslim, whose reports the scholars accept as authentic.