(73) باب مَنْ كَفَّرَ أَخَاهُ بِغَيْرِ تَأْوِيلٍ فَهْوَ كَمَا قَالَ
Narrated Thabit bin Ad-Dahhak:
The Prophet (ﷺ) said, "Whoever swears by a religion other than Islam (i.e. if he swears by saying that he is a non-Muslim in case he is telling a lie), then he is as he says if his oath is false and whoever commits suicide with something, will be punished with the same thing in the (Hell) fire, and cursing a believer is like murdering him, and whoever accuses a believer of disbelief, then it is as if he had killed him."
Reference: Sahih al-Bukhari 6105
In-book reference: Book 78, Hadith 132
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 8, Book 73, Hadith 126