The Gemara in Masechet Berachot (30) establishes the obligation of "Tefilat Ha'derech," the special Beracha recited when embarking on a trip. There is a debate among the Rabbis in the Gemara as to whether or not this prayer must be recited while standing. Rav Hisda maintained that one must stop traveling and stand while reciting Tefilat Ha'derech, while Rav Sheshat was of the opinion that one may sit and continue traveling as he recites this prayer. The Rishonim (Medieval Halachic scholars) disagree in determining the final Halacha. Tosafot (commentaries by French and German Medieval scholars) followed the lenient position of Rav Sheshat, whereas the Rif (Rabbi Yishak of Fez, Morocco, 1013-1103) accepted the stringent ruling of Rav Hisda. The Shulhan Aruch (Orah Haim 110; listen to audio recording for precise citation) ruled that optimally, one should stop traveling and stand for the recitation of Tefilat Ha'derech. If, however, this is not possible, one may certainly rely on the lenient view of Rav Sheshat and recite the prayer while seated. If a person forgot to recite Tefilat Ha'derech at the beginning of his trip, until what point may he still recite the prayer? Halacha follows the position of the Behag ("Ba'al Halachot Gedolot") that allows a person to recite Tefilat Ha'derech even in the middle of his trip, so long as at least 72 minutes of traveling remain. For example, if a person takes a four-hour flight and remembered two hours into the flight that he had not recited Tefilat Ha'derech, he may recite the prayer at that point, since more than 72 minutes of traveling remain. If, however, a person did not remember to recite Tefilat Ha'derech until he is within 72 minutes of his destination, then he recites the Beracha but omits Hashem's Name in the conclusion. Thus, instead of reciting "Baruch Ata Hashem Shome'a Tefila," he would say, "Baruch Ata Shome'a Tefila." Summary: A traveler should preferably recite Tefilat Ha'derech while standing, but if this is not possible he may recite the prayer while sitting. A person who forgot to recite Tefilat Ha'derech at the beginning of his trip may recite it later, so long as at least 72 minutes of travel time remain. Otherwise, he may recite the Beracha but he must omit Hashem's Name from the concluding phrase, "Baruch Ata Hashem Shome'a Tefila."