The Prophet [SAW] said: "The extinction of the whole world is less significant before Allah than killing a Muslim man." (Using translation from Nasāʾī 3987)
أَنَّ النَّبِيِّ ﷺ قَالَ «لَزَوَالُ الدُّنْيَا أَهْوَنُ عِنْدَ اللهِ مِنْ قَتْلِ رَجُلٍ مُسْلِمٍ»