Musnad Ahmad 830:11

Waki’ narrated to us, Sufyan narrated to us, from Yunus ibn ‘Ubayd, from al-Hakam ibn al-A’raj, from al-Ash’ath ibn 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 to him, that he should smell its fragrance."