What is the ruling on a traveler performing prayers by combining them in advance (jama' taqdim)? And is he obligated to go to the mosque and perform the prayer again—as a supererogatory prayer (nafilah)—if he hears the adhan for the second prayer?

Combining two prayers is a concession for the traveler, whether he is on the move or has dismounted. However, performing each prayer at its appointed time is preferable, due to the practice of the Prophet (peace be upon him) and to avoid scholarly اختلاف (disagreement). Thus, whoever combines prayers is not required to repeat the Asr prayer if its time enters and he attends the congregational prayer, because his obligation has been fulfilled and because the Prophet (peace be upon him) forbade performing a single prayer twice. The mentioned hadith applies specifically to someone who prayed in his home and then came to the mosque.