Al-Kāfi - Volume 1 > The Conditions of the Imams in the Matters of Age
Hadith #8

8ـ عَلِيُّ بْنُ إِبْرَاهِيمَ عَنْ أَبِيهِ قَالَ قَالَ عَلِيُّ بْنُ حَسَّانَ لابي جعفر (عَلَيْهِ السَّلام) يَا سَيِّدِي إِنَّ النَّاسَ يُنْكِرُونَ عَلَيْكَ حَدَاثَةَ سِنِّكَ فَقَالَ وَمَا يُنْكِرُونَ مِنْ ذَلِكَ قَوْلَ الله عَزَّ وَجَلَّ لَقَدْ قَالَ الله عَزَّ وَجَلَّ لِنَبِيِّهِ (صَلَّى اللهُ عَلَيْهِ وَآلِه) قُلْ هذِهِ سَبِيلِي أَدْعُوا إِلَى الله عَلى‏ بَصِيرَةٍ أَنَا وَمَنِ اتَّبَعَنِي فَوَ الله مَا تَبِعَهُ إِلا علي (عَلَيْهِ السَّلام) وَلَهُ تِسْعُ سِنِينَ وَأَنَا ابْنُ تِسْعِ سِنِينَ.

8. Ali ibn Ibrahim has narrated from his father who has said the following. “Once abu Ali ibn Hassa’n spoke to abu Ja‘far (a.s.) saying, “My mater, people refuse to accept your Imamat (Leadership with Divine Authority) because of you very young age.” He said, “Why do they not refuse to accept the words of Allah, the Most Majestic, the Most gracious. Allah, the Most Majestic, the Most gracious, has said to His prophet, “ (Muhammad), say, “This is my way. I and all my followers invite you to God with proper understanding.. .” (12:108) By Allah, only Ali (a.s.) followed him at that time and he was only nine years old. I also am nine years old.