Musnad Ahmad 830:25

Isma'il narrated to us, Yunus bin 'Ubayd narrated to us, from al-Hakam bin al-A'raj, from al-Ash'ath bin Tharmalah, from Abu Bakrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever kills a person under a covenant (mu'ahadah) without its justification, Allah has forbidden Paradise for him, [such that he shall not] smell its fragrance."