Sunan At-Tirmidhi

Abu Hurairah narrated:""Al-Aqra' bin Habis saw the prophet kissing Al-Hasan""- Ibn Abi 'Umar (one of the narrators) said:""Al-Hasan and Al-Husain."" So he said: 'I have ten children and I do not kiss any one of them.' So the Messenger of Allah said: 'Whoever shows no mercy, he will be shown no mercy