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 3956

Narrated Al-Bara:

I and Ibn `Umar were considered too young (to take part) in the battle of Badr, and the number of the Emigrant warriors were over sixty (men) and the Ansar were over 249.

حَدَّثَنِي مَحْمُودٌ، حَدَّثَنَا وَهْبٌ، عَنْ شُعْبَةَ، عَنْ أَبِي إِسْحَاقَ، عَنِ الْبَرَاءِ، قَالَ اسْتُصْغِرْتُ أَنَا وَابْنُ عُمَرَ يَوْمَ بَدْرٍ، وَكَانَ الْمُهَاجِرُونَ يَوْمَ بَدْرٍ نَيِّفًا عَلَى سِتِّينَ، وَالأَنْصَارُ نَيِّفًا وَأَرْبَعِينَ وَمِائَتَيْنِ‏.‏
Reference : Sahih al-Bukhari 3956
In-book reference : Book 64, Hadith 8
USC-MSA web (English) reference : Vol. 5, Book 59, Hadith 292
  (deprecated numbering scheme)