Sahih Bukhari
Good Manners and Form (Al-Adab)
Volume: 008 / Book Number: 073 / Hadith Number: 26
Narrated Abu Huraira:
Allah's Apostle kissed Al-Hasan bin Ali while Al-Aqra' bin Habis At-Tamim was sitting beside him. Al-Aqra said, "I have ten children and I have never kissed anyone of them," Allah's Apostle cast a look at him and said, "Whoever is not merciful to others will not be treated mercifully."