Good Manners and Form (Al-Adab)
Ṣaḥīḥ al-Bukhārī 6166
وَيْلَكُمْ ـ أَوْ وَيْحَكُمْ قَالَ شُعْبَةُ شَكَّ هُوَ ـ لاَ تَرْجِعُوا بَعْدِي كُفَّارًا، يَضْرِبُ بَعْضُكُمْ رِقَابَ بَعْضٍ
waylakum aw wayḥakum qāla shuʿbah shakka huwa lā tarjiʿūā baʿdī kuffāran, yaḍribu baʿḍukum riqāba baʿḍin
Transliteration generated from the Arabic vowel marks
The full narration
Narrated Ibn `Umar:The Prophet (ﷺ) said, "Wailakum" (woe to you) or "waihakum" (May Allah be merciful to you)." Shu`ba is not sure as to which was the right word. "Do not become disbelievers after me by cutting the necks of one another
Source
Ṣaḥīḥ al-Bukhārī · 6166
Ṣaḥīḥ