Hadith: Whoever does not show mercy will not be shown mercy

عن أبي هريرة رضي الله عنه قال: قَبَّلَ النبيُّ صلى الله عليه وسلم الحسنَ بنَ عليٍّ رضي الله عنهما ، وعنده الأَقْرَعُ بنُ حَابِسٍ، فقال الأَقْرَعُ: إنَّ لي عَشَرَةً من الوَلَدِ ما قَبَّلْتُ مِنْهُمْ أَحَدًا، فَنَظَرَ إليهِ رسولُ اللهِ صلى الله عليه وسلم فقال: «مَنْ لَا يَرْحَم لَا يُرْحَمُ!».
[صحيح] - [متفق عليه]

Abu Hurayrah (may Allah be pleased with him) reported: The Messenger of Allah (may Allah’s peace and blessings be upon him) kissed Al-Hasan ibn ‘Ali while Al-Aqra‘ ibn Hābis was present. Al-Aqra‘ observed: "I have ten children, and I have never kissed any of them." Thereupon, the Messenger of Allah (may Allah’s peace and blessings be upon him) looked at him and said: "Whoever does not show mercy will not be shown mercy!"
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

Abu Hurayrah (may Allah be pleased with him) informed that the Messenger (may Allah's peace and blessings be upon him) kissed Al-Hasan ibn ‘Ali while Al-Aqra‘ ibn Hābis At-Tamīmi was sitting with him. Al-Aqra‘ remarked: "I have ten children, and I have never kissed any of them." Thereupon, the Messenger (may Allah's peace and blessings be upon him) looked at him and said: "Whoever does not show mercy will not be shown mercy." In another narration: "What can I do if Allah has removed mercy from your heart?" Meaning: If Allah, the Almighty, has taken the feeling of mercy away from your heart, can I ever bring it back?