Sayyiduna Ubadah bin al-Samit (may Allah be pleased with him) reports this saying of the Prophet (peace and blessings be upon him): "Every sin, it may be that Allah Almighty will forgive it, except (the sin of) one who dies as a polytheist or who deliberately kills a believer."
Al-Bazzar recorded it, and its narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الفتن - أعاذنا الله منها / 12298