Sahih Bukhari
Good Manners and Form (Al-Adab)
Volume: 008 / Book Number: 073 / Hadith Number: 27
Narrated 'Aisha:
A bedouin came to the Prophet and said, "You (people) kiss the boys! We don't kiss them." The Prophet said, "I cannot put mercy in your heart after Allah has taken it away from it."