The Messenger of Allah ﷺ said, "Whoever swears by a religion other than Islam, he is as he has said. The son of Adam is not bound by a vow concerning what he does not possess. Whoever kills himself with something in this world will be tormented with it on the Day of Resurrection. Whoever curses a believer, it is as if he had killed him, and whoever accuses a believer of disbelief, it is as if he had killed him."
أن رسول الله ﷺ قال: من حلف على ملة غير الإسلام فهو كما قال، وليس على ابن آدم نذر فيما لا يملك، ومن قتل نفسه بشيء في الدنيا عذب به يوم القيامة، ومن لعن مؤمنا فهو كقتله، ومن قذف مؤمنا بكفر فهو كقتله.
Bukhari narrated it in Adab (6047), and Muslim in Iman (110), both through Yahya bin Abu Kathir, from Abu Qilaba, that Thabit bin Dahhak mentioned it.His saying, "like killing him," means it is among the major sins, because killing a believer without right is one of the major sins. Salama bin Akwa said, "When we saw a man cursing his brother, we considered that he had entered through one of the doors of the major sins."Tabarani narrated it in Awsat, Majma Bahrayn (3141), from Muhammad bin Hasan, who said, Yazid bin Mawhab told us, Ibn Wahb told us, Amr bin Harith informed me, that Bukayr bin Abdullah bin Ashaj told him, that Yazid bin Abu Ubayd, the freedman of Salama bin Akwa, told him that he heard Salama bin Akwa say it, and he mentioned it as a stopped narration (mawquf), and its chain of transmission is sahih.Muhammad bin Hasan, the teacher of Tabarani, is Ibn Qutayba. Abu Bakr Ibn Muqri said, "the sheikh of Sham," and Daraqutni deemed him reliable.