Musnad Ahmad 30:3542

Rawh narrated to us, Muhammad ibn Abi Hafsah narrated to us, from Ibn Shihab, from Abu Salamah, from Abu Hurairah, that the Messenger of Allah (peace and blessings of Allah be upon him) used to kiss Al-Hasan ibn Ali. Al-Aqra’ ibn Habis said: "I have ten children, and I have not kissed any one of them." The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever does not show mercy, will not be shown mercy."