Abu Adrees says that I heard Sayyiduna Muawiyah (may Allah be pleased with him), who narrated very few hadiths, saying: I heard the Prophet (peace and blessings of Allah be upon him) say: It is hoped that Allah Almighty will forgive every sin, except for the one who dies in a state of disbelief or the person who deliberately kills a Muslim.
Hadith Referenceمسند احمد / مسند الشاميين / 16907
Hadith Gradingحکم دارالسلام:صحيح لغيره، وهذا إسناد حسن