Sahih Bukhari Volume 007, Book 072, Hadith Number 772.

Narrated By Abu Huraira : I was with Allah's Apostle in one of the Markets of Medina. He left (the market) and so did I. Then he asked thrice, "Where is the small (child)?" Then he said, "Call Al-Hasan bin 'Ali." So Al-Hasan bin 'Ali got up and started walking with a necklace (of beads) around his neck. The Prophet stretched his hand out like this, and Al-Hasan did the same. The Prophet embraced him and said, "0 Allah! l love him, so please love him and love those who love him." Since Allah's Apostle said that. nothing has been dearer to me than Al-Hasan.