Hadithcore
Sahih Muslim, 1868 b
sahih

This tradition has been handed down through a different chain Of transmitters with the following change in the wording:

" I was fourteen years old and he thought me too young (to participate in the fight)."

وَحَدَّثَنَاهُ أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا عَبْدُ اللَّهِ بْنُ إِدْرِيسَ، وَعَبْدُ الرَّحِيمِ بْنُ، سُلَيْمَانَ ح وَحَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، حَدَّثَنَا عَبْدُ الْوَهَّابِ، - يَعْنِي الثَّقَفِيَّ - جَمِيعًا عَنْ عُبَيْدِ، اللَّهِ بِهَذَا الإِسْنَادِ غَيْرَ أَنَّ فِي حَدِيثِهِمْ وَأَنَا ابْنُ أَرْبَعَ عَشْرَةَ سَنَةً فَاسْتَصْغَرَنِي ‏.‏

References2 variants
In-Book Reference
Book 33, Hadith 134
USC-MSA web (English) reference
Book 20, Hadith 4606 (deprecated numbering scheme)