Musnad Ahmad 30:531

'Abd al-Razzaq narrated to us, Ma'mar informed us, from al-Zuhri, Abu Salamah narrated to me, from Abu Hurairah, that the Messenger of Allah (may Allah bless him and grant him peace) kissed al-Hasan ibn 'Ali (may Allah be pleased with them both), while al-Aqra' ibn Habis al-Tamimi was sitting. Al-Aqra' said: "O Messenger of Allah, I have ten children, and I have never kissed a single one of them." The Messenger of Allah (may Allah bless him and grant him peace) looked at him and said: "Indeed, whoever does not show mercy will not be shown mercy."