Sahih al-Bukhari, Volume 5, Book 59, Hadith 292 (USC-MSA) ⓘ
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.
also referenced under: Book 64, Hadith 8 (In-Book) & Hadith 3956 (DarusSalam)