Whoever does not show mercy will not be shown mercy
Authentic hadithعن 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!».
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!"
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?
Source · Narrated by Bukhari & Muslim
From the Hadith Encyclopedia (hadeethenc.com).