It is narrated from Sayyiduna Abu Bakrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever unjustly kills a mu'ahad (one with whom a covenant has been made), Allah Almighty will make Paradise forbidden for him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجزية / 18849