Al-Kāfi - Volume 1 > The Conditions of the Imams in the Matters of Age
Hadith #5

5ـ سَهْلُ بْنُ زِيَادٍ عَنْ عَلِيِّ بْنِ مَهْزِيَارَ عَنْ مُحَمَّدِ بْنِ إِسْمَاعِيلَ بْنِ بَزِيعٍ قَالَ سَأَلْتُهُ يَعْنِي أَبَا جَعْفَرٍ (عَلَيْهِ السَّلام) عَنْ شَيْ‏ءٍ مِنْ أَمْرِ الامَامِ فَقُلْتُ يَكُونُ الامَامُ ابْنَ أَقَلَّ مِنْ سَبْعِ سِنِينَ فَقَالَ نَعَمْ وَأَقَلَّ مِنْ خَمْسِ سِنِينَ فَقَالَ سَهْلٌ فَحَدَّثَنِي عَلِيُّ بْنُ مَهْزِيَارَ بِهَذَا فِي سَنَةِ إِحْدَى وَعِشْرِينَ وَمِائَتَيْنِ.

5. Sahl ibn Ziyad has narrated from Ali ibn Mahziyar from Muhammad ibn ’Isma‘il ibn Bazi‘ who has said the following. “I asked abu Ja‘far (a.s.) of an issue of the Imamat (Leadership with Divine Authority) and I said, “Can the Imam be a boy less than seven years old?” He said, “Yes, he can even be less the five years old.” Sahl has said that ibn Mahziyar narrated this to me in the year two hundred twenty one.”