The Prophet ﷺ used to stick out his tongue for Husayn, and the child would see the redness of his tongue and become delighted with him.Uyayna bin Badr said to him, "Do I really see him doing this with him? By Allah, I have children whose beards have grown, yet I have never kissed any of them."The Prophet ﷺ replied, "Whoever does not show mercy will not be shown mercy."
كان النبي ﷺ يدلع لسانه للحسين، فيرى الصبي حمرة لسانه، فيهش إليه، فقال له عيينة بن بدر: ألا أراه يصنع هذا بهذا، فوالله، إنه ليكون لي الولد، قد خرج وجهه، وما قبلته قط، فقال النبي ﷺ: من لا يرحم لا يرحم.
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Ibn Hibban (5596) narrated it via Khalid bin Abdullah Wasiti, and Abu Shaykh in Akhlaq al-Nabi ﷺ (p. 86) via Muhammad bin Bishr, both of them from Muhammad bin Amr bin Alqama, from Abu Salama, from Abu Hurairah, who said, then he mentioned it. Its chain is hasan because of Muhammad bin Amr bin Alqama, for he is hasan al-hadith.However, Hannad narrated it in Zuhd (1330) from Abda, and Abu Ubayd narrated it in Ghareeb al-Hadith (3/144) from Yazid bin Harun, both of them from Muhammad bin Umar bin Alqama, from Abu Salama, as a mursal narration, and neither of them mentioned "Abu Hurairah." The connected version is closer to being correct because of its corroborating evidence.