What is the ruling on someone who prayed Asr while traveling and then discovered that the time for Asr had not yet begun? Is he obligated to repeat the prayer?

If a traveler prays 'Asr as a jama' taqdim (combining it with Dhuhr), he is not required to repeat it if he arrives before the 'Asr time. This is because he combined it in a state that permits combining, whether he was certain of arriving before 'Asr time or not. This is similar to the lack of obligation to repeat for someone who prayed with tayammum and then found water before the time expired. So, the Dhuhr time for a traveler is the time for both Dhuhr and 'Asr.