What is the ruling regarding a man telling his wife, "I will divorce you tomorrow," as a joke?

One must be cautious about joking regarding divorce, as it is a matter in which seriousness and jest are treated equally, due to the Prophet's (peace be upon him) saying: "There are three matters, serious in which is serious, and jest in which is serious: marriage, divorce, and revocation [of divorce]." Scholars have agreed that an explicit utterance of divorce obliges the one who utters it, even if he claims he was joking. However, a promise of divorce does not constitute divorce, and there is no desire to enforce it.