Language:
كتاب جزاء الصيد
28  
Penalty of Hunting while on Pilgrimage
(25)
Chapter: The Hajj of boys (children etc.)
(25)
باب حَجِّ الصِّبْيَانِ
Sahih al-Bukhari 1858

Narrated As-Sa'ib bin Yazid:

(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)

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.‏
Reference : Sahih al-Bukhari 1858
In-book reference : Book 28, Hadith 38
USC-MSA web (English) reference : Vol. 3, Book 29, Hadith 82
  (deprecated numbering scheme)