Musnad Ahmad 830:150

'Abd al-Razzaq narrated to us, Sufyan informed us, from Yunus ibn 'Ubayd, from al-Hakam ibn al-A'raj, from al-Ash'ath ibn Tharmala, 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 their right, Allah has forbidden Paradise to him, [such that he shall not] smell its fragrance."