What is the ruling regarding someone upon whom Fajr prayer is due before he has prayed 'Isha, and is it obligatory to pray 'Isha first or Fajr?

It is not permissible to delay prayer beyond its appointed time without a valid excuse, for Allah Almighty says: "Indeed, prayer has been decreed upon the believers a [definite] time." And whoever delays the 'Isha' prayer until Fajr has indeed caused it to pass its time, making it a missed prayer. Scholars have differed on the obligation of praying the missed prayer before the current one. If the adhan occurs before true dawn, then the 'Isha' prayer must be made up before its time expires.