The Prophet ﷺ said, "Cursing a believer is like killing him. Whoever kills himself with something in this world will be tortured with it on the Day of Resurrection. A Muslim is not bound by a vow concerning what he does not own. Whoever accuses a believer of disbelief, it is as though he has killed him. And whoever swears falsely by a religion other than Islam is as he has said."
حدثنا يحيى بن سعيد قال حدثنا هشام ويزيد قال أخبرنا هشام قال حدثني يحيى عن أبي قلابة
أن النبي ﷺ قال لعن المؤمن كقتله ومن قتل نفسه بشيء في الدنيا عذب به في الآخرة وليس على رجل مسلم نذر فيما لا يملك ومن رمى مؤمنا بكفر فهو كقتله ومن حلف بملة سوى الإسلام كاذبا فهو كما قال
Its isnad is sahih, and its narrators are well known. The hadith was narrated by Bukhari 10/464, no. 6047, Fath al-Bari, in the Book of Manners, chapter on what is forbidden regarding abuse, and by Tirmidhi 5/22, no. 2631, in the Book of Faith, chapter on what has been said about one who accuses his brother of disbelief.