Sayyiduna Abdullah bin Amr (may Allah be pleased with them both) said: Killing a believer is a graver matter in the sight of Allah than the passing away of the world. This is the preserved (mahfuz) version, and it is mawquf (stopping at the Companion).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15869