Aqra bin Habis saw the Prophet ﷺ kissing Husayn and said, "I have ten children, and I have never done this to any of them."The Messenger of Allah ﷺ replied, "Whoever shows no mercy will be shown no mercy."
حدثنا مسدد حدثنا سفيان عن الزهري عن أبي سلمة
أن الأقرع بن حابس أبصر النبي ﷺ وهو يقبل حسينا فقال إن لي عشرة من الولد ما فعلت هذا بواحد منهم فقال رسول الله ﷺ من لا يرحم لا يرحم
Sourced from Sahih Muslim, Book of Virtues, Chapter on his mercy toward children and dependents and his humility, and the merit of that, no. 2318, from the hadith of Sufyan, and Bukhari, Book of Manners, Chapter on mercy toward children, kissing them, and embracing them, no. 5997, from the hadith of Zuhri, with the same wording.