Sahih Muslim
The Book Pertaining to the Excellent Qualities of the Holy Prophet (may Peace be upon them) and His Companions (Kitab Al-Fadail)
Book Number: 030 / Hadith Number: 5736
Abu Huraira reported that al-Aqra' b. Habis saw Allah's Apostle (may peace be upon him) kissing Hasan. He said: I have ten children, but I have never kissed any one of them, whereupon Allah's Messenger (may peace be upon him) said: He who does not show mercy (towards his children), no mercy would be shown to him.