The pre-Tribulational Rapture of the Church is found throughout the New Testament, so the short answer is that God wrote this prophecy.
Please, DON'T let anyone tell you that the word "rapture" isn't found in the Bible (it is, but it's derived from a word in the Latin Vulgate translation), or that the Rapture wasn't even thought of until the 1900's (we have ancient Christian manuscripts that mention the Rapture).
Since there are so many verses in the New Testament that teach this event, there isn't enough space here to go into it. However, Dr. Arnold Fruchtenbaum, a Messianic Jewish scholar, has written a paper on the subject which you can download and print free here: http://www.arielm.org/dcs/pdf/mbs039m.pd…
EDIT: @ Sirius: there is no mention of the Rapture in the OT because the Rapture is a promise made to the CHURCH, not to those living before Christ. And before you say "then it's not valid," may I remind you that Paul himself wrote that the Church was a MYSTERY not revealed until NT times (Col. 1:24-27)?