Al-Kāfi - Volume 3 > Salah (Prayer) of Children and When Should They Begin Salah (Prayer)
Hadith #3

3ـ الْحُسَيْنُ بْنُ مُحَمَّدٍ عَنْ مُعَلَّى بْنِ مُحَمَّدٍ عَنِ الْوَشَّاءِ عَنِ الْمُفَضَّلِ بْنِ صَالِحٍ عَنْ جَابِرٍ عَنْ أَبِي جَعْفَرٍ (عَلَيْهِ الْسَّلام) قَالَ سَأَلْتُهُ عَنِ الصِّبْيَانِ إِذَا صَفُّوا فِي الصَّلاَةِ الْمَكْتُوبَةِ قَالَ لاَ تُؤَخِّرُوهُمْ عَنِ الصَّلاَةِ الْمَكْتُوبَةِ وَفَرِّقُوا بَيْنَهُمْ.

3. Al-Husayn ibn Muhammad has narrated from Mu‘alla’ ibn Muhammad from al-Washsha’ from al-Mufaddal ibn Salih from Jabir who has said the following: “I once asked abu Ja‘far, ‘Alayhi al-Salam, about children when they line up for obligatory Salah (prayer). He (the Imam) said, ‘Do not push them back from obligatory but separate them from each other (boys from girls).’”