كتاب المغازى
64  
Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi)
(6)
Chapter: The number of the warriors of Badr
(6)
باب عِدَّةِ أَصْحَابِ بَدْرٍ
Sahih al-Bukhari 3955

Narrated Al-Bara:

I and Ibn `Umar were considered too young to take part in the battle of Badr.

حَدَّثَنَا مُسْلِمٌ، حَدَّثَنَا شُعْبَةُ، عَنْ أَبِي إِسْحَاقَ، عَنِ الْبَرَاءِ، قَالَ اسْتُصْغِرْتُ أَنَا وَابْنُ، عُمَرَ‏.‏
Reference : Sahih al-Bukhari 3955
In-book reference : Book 64, Hadith 7
USC-MSA web (English) reference : Vol. 5, Book 59, Hadith 291
  (deprecated numbering scheme)