Sunan an-Nasa'i, Volume 5, Book 37, Hadith 4028 (DarusSalam) ⓘ
It was narrated that Usamah bin Sharik said: "The Messenger of Allah [SAW] said: 'Any man who goes out and tries to create division among my Ummah, strike his neck (kill him).'"
أَخْبَرَنَا مُحَمَّدُ بْنُ قُدَامَةَ، قَالَ حَدَّثَنَا جَرِيرٌ، عَنْ زَيْدِ بْنِ عَطَاءِ بْنِ السَّائِبِ، عَنْ زِيَادِ بْنِ عِلاَقَةَ، عَنْ أُسَامَةَ بْنِ شَرِيكٍ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " أَيُّمَا رَجُلٍ خَرَجَ يُفَرِّقُ بَيْنَ أُمَّتِي فَاضْرِبُوا عُنُقَهُ " .
also referenced under: Hadith 4023 (Reference) & Book 37, Hadith 58 (In-Book)