Sahih al-Bukhari 3955
Sahih al-Bukhari · Book 64: Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi) · The number of the warriors of Badr
Narrated Al-Bara:
حَدَّثَنَا مُسْلِمٌ ، حَدَّثَنَا شُعْبَةُ ، عَنْ أَبِي إِسْحَاقَ ، عَنِ الْبَرَاءِ ، قَالَ اسْتُصْغِرْتُ أَنَا وَابْنُ، عُمَرَ.
I and Ibn `Umar were considered too young to take part in the battle of Badr.