Abdullah bin Umar reported,

The Prophet ﷺ reviewed him on the day of Uhud when he was fourteen years old and did not permit him to join the battle, then reviewed him again on the day of the Trench when he was fifteen years old and permitted him.

Bukhari 4097

Sahih

حدثنا يعقوب بن إبراهيم حدثنا يحيى بن سعيد عن عبيد الله قال أخبرني نافع

عن ابن عمر رضي الله عنهما

أن النبي ﷺ عرضه يوم أحد وهو ابن أربع عشرة سنة فلم يجزه وعرضه يوم الخندق وهو ابن خمس عشرة سنة فأجازه

البخاري 4097صحيح
Contents
Reference
Commentary
Ayat
Collection
Sahih Bukhari
Book
Expeditions
Chapter
Battle of Khandaq, which is Ahzab
Numbering
Darussalam 4097
Bukhari 4097