Musnad Ahmad 1138:3

Ishaq ibn Isa narrated to us, Malik informed me, from al-Ala, from Ma’bad ibn Ka’b ibn Malik, from his brother Abdullah ibn Ka’b, from Abu Umamah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever usurps the right of a Muslim by his oath, Allah has forbidden Paradise for him and made the Fire mandatory for him." They said: "Even if it is a trivial thing?" He said: "Even if it is a twig of Arak," he said it three times.