Narrated Thabit bin Ad-Dahhak:
The Prophet (p.b.u.h) 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.”
[Bukhari Vol 8 Book 73, Hadith 126]