Musnad Ahmad 1138:1

Yazid ibn Harun narrated to us, Muhammad ibn Ishaq narrated to us, from Ma'bad ibn Ka'b ibn Malik, from his brother Abdullah ibn Ka'b ibn Malik, from Abu Umamah ibn Sahl, one of the Banu Harithah, [who said]: I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "No man shall usurp the right of a Muslim man by his oath, except that Allah has made Paradise forbidden to him and has made the Fire mandatory for him." A man said: "O Messenger of Allah, even if it is a trivial thing?" He said: "Even if it is a *siwak* (tooth-stick) made of *arak* wood."