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Jarir reported that the Prophet, may Allah bless him and grant him peace, said, "Allah will not show mercy to a person who does not show mercy to other people."
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مُسَدَّدٌ، قَالَ: حَدَّثَنَا يَحْيَى، عَنْ إِسْمَاعِيلَ قَالَ: أَخْبَرَنِي قَيْسٌ قَالَ: أَخْبَرَنِي جَرِيرٌ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ: مَنْ لا يَرْحَمُ النَّاسَ لا يَرْحَمُهُ اللَّهُ.
sunnah.com