Sahih al-Bukhari 1858
Sahih al-Bukhari · Book 28: Penalty of Hunting while on Pilgrimage · The Hajj of boys (children etc.)
Narrated As-Sa'ib bin Yazid:
حَدَّثَنَا عَبْدُ الرَّحْمَنِ بْنُ يُونُسَ ، حَدَّثَنَا حَاتِمُ بْنُ إِسْمَاعِيلَ ، عَنْ مُحَمَّدِ بْنِ يُوسُفَ ، عَنِ السَّائِبِ بْنِ يَزِيدَ ، قَالَ حُجَّ بِي مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم وَأَنَا ابْنُ سَبْعِ سِنِينَ.
(While in the company of my parents) I was made to perform Hajj with Allah's Messenger (ﷺ) and I was a seven-year-old boy then. (Fath-ul-Bari, p.443, Vol.4)