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Abu Hurayra reported that the Messenger of Allah, may Allah bless him and grant him peace, said, "The person who is strong is not strong because he can knock people down. The person who is strong is the one who controls himself when he is angry."
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا إِسْمَاعِيلُ قَالَ: حَدَّثَنِي مَالِكٌ، عَنِ ابْنِ شِهَابٍ، عَنْ سَعِيدِ بْنِ الْمُسَيِّبِ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللهِ صلى الله عليه وسلم قَالَ: لَيْسَ الشَّدِيدُ بِالصُّرَعَةِ، إِنَّمَا الشَّدِيدُ الَّذِي يَمْلِكُ نَفْسَهُ عِنْدَ الْغَضَبِ.
sunnah.com