Sahih Muslim, Chapter: Faith (Kitab Al Iman)

reported that he pledged allegiance to the Messenger of Allah (may peace be upon him) under the Tree, and verily the Messenger of Allah (may peace be upon him) observed: He who took an oath of a religion other than Islam, in the state of being a liar, would became so, as he professed. He who killed himself with a thing would be tormented on the Day of Resurrection with that very thing. One is not obliged to offer votive offering of a thing which is not in his possession.