It is narrated from Sayyiduna Thabit bin Dhahak that the Prophet (peace be upon him) said: Cursing a Muslim is like killing him, and whoever kills someone with something in this world will be punished with the same thing in the Hereafter. And there is no vow upon a Muslim man that is beyond his capacity. And whoever accuses a Muslim of disbelief, it is like killing him. And whoever swears a false oath by any religion other than Islam, he will be considered as he has said (for example, Qadiani, Hindu, or Christian).
Hadith Referenceمسند احمد / مسند المدنيين رضي الله عنهم اجمعين / 16385