Sahih Bukhari Volume 8, Book 73, Hadith Number 26.

Narated By 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.”