(29) باب غَزْوَةُ الْخَنْدَقِ وَهْىَ الأَحْزَابُ
Narrated Ibn `Umar:
That the Prophet (ﷺ) inspected him on the day of Uhud while he was fourteen years old, and the Prophet (ﷺ) did not allow him to take part in the battle. He was inspected again by the Prophet (ﷺ) on the day of Al- Khandaq (i.e. battle of the Trench) while he was fifteen years old, and the Prophet (ﷺ) allowed him to take Part in the battle.
Reference: Sahih al-Bukhari 4097
In-book reference: Book 64, Hadith 141
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 5, Book 59, Hadith 423