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Abu Hurayra reported that the Prophet, may Allah bless him and grant him peace, said, "When two people revile each other, the responsibility for what they say rests on the first to speak as long as the one who wronged does not become excessive."
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا إِبْرَاهِيمُ بْنُ مُوسَى، قَالَ: حَدَّثَنَا إِسْمَاعِيلُ بْنُ جَعْفَرٍ، قَالَ: حَدَّثَنَا الْعَلاَءُ بْنُ عَبْدِ الرَّحْمَنِ، عَنْ أَبِيهِ، عَنْ أَبِي هُرَيْرَةَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ: الْمُسْتَبَّانِ مَا قَالاَ فَعَلَى الْبَادِئِ، مَا لَمْ يَعْتَدِ الْمَظْلُومُ.
sunnah.com