If a traveller starts the third Rak’at in his Qaṣr Ṣalāĥ then there are two possibilities:
1. If he has already sat for the Qa’daĥ Akhīraĥ (i.e., final sitting) [after the second Rak’at] for as long as it takes to recite the Tashaĥĥud, he must revert to the Qa’daĥ position provided he has not yet offered the Sajdaĥ of the third Rak’at. He is then required to perform Sajdaĥ Saĥw and Salām [to finish his Ṣalāĥ]. And if he did not revert and offered Salām while standing, even then his Ṣalāĥ would be valid, but a Sunnaĥ would get missed.
If he has offered Sajdaĥ of the third Rak’at then he must add another Rak’at [to make it four] and finish it after performing Sajdaĥ Saĥw. In this case, the last two Rak’āt will be regarded as Nafl.
2. If he has stood up without sitting for the Qa’daĥ Akhīraĥ [after the second Rak’at] then as long as he has not offered the Sajdaĥ of the third Rak’at, he must revert to the Qa’daĥ, perform Sajdaĥ Saĥw and then offer Salām.
If he has offered the Sajdaĥ of the third Rak’at [without sitting in Qa’daĥ after the second Rak’at, then his] Farḍ will become invalid. Now, he should add another Rak’at