TABARANI 10544:
from ʿAbdullāh b. Masʿūd —
[Machine] The Messenger of Allah ﷺ said: "There is no two Muslims except that there is a barrier between them from Allah. So if one of them says to his companion 'Hujr' (meaning boycott or severance), he has violated the barrier. And if one of them says 'O disbeliever', then he has committed disbelief."
الطبراني ١٠٥٤٤:
عن عبد الله بن مسعود —
قَالَ رَسُولُ اللهِ ﷺ مَا مِنْ مُسْلِمَيْنِ إِلَّا وَبَيْنَهُمَا سِتْرٌ مِنَ اللهِ فَإِذَا قَالَ أَحَدُهُمَا لِصَاحِبِهِ هُجْرًا هَتَكَ سِتْرَهُ وَإِذَا قَالَ يَا كَافِرُ فَقَدْ كَفَرَ أَحَدُهُمَا
hadithunlocked.com