It is narrated from Sayyiduna Mu'awiyah bin Abi Sufyan (may Allah be pleased with him) that he heard the Messenger of Allah (peace and blessings be upon him) say: "There is no sin but that it is hoped Allah will forgive it, except the one who is a polytheist or who kills a believer deliberately."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16905