Al-Kāfi - Volume 5 > The Age When It Is Lawful to Kiss a Girl
Hadith #2

2- حُمَيْدُ بْنُ زِيَادٍ عَنِ الْحَسَنِ بْنِ مُحَمَّدِ بْنِ سَمَاعَةَ عَنْ غَيْرِ وَاحِدٍ عَنْ أَبَانِ بْنِ عُثْمَانَ عَنْ عَبْدِ الرَّحْمَنِ بْنِ يَحْيَى عَنْ زُرَارَةَ عَنْ أَبِي عَبْدِ اللهِ (عَلَيْهِ السَّلام) قَالَ قَالَ إِذَا بَلَغَتِ الْجَارِيَةُ الْحُرَّةُ سِتَّ سِنِينَ فَلا يَنْبَغِي لَكَ أَنْ تُقَبِّلَهَا.

2. Humayd ibn Ziyad has narrated from al-Hassan ibn Muhammad ibn Sama‘ah from more than one person from Aban ibn ‘Uthman from ‘Abd al-Rahman ibn Yahya from Zurarah who has said the following: “Abu ‘Abd Allah, ‘Alayhi al-Salam, has said, ‘When a free girl becomes six years old then it is not proper for you to kiss her.’”