Is it permissible to combine the Isha prayer with Maghrib due to the inability to perform it on time, or is it better to make it up with the Fajr prayer?

Allah has ordained prayer at specific times, as He states: "Indeed, prayer has been decreed upon the believers a timed ordinance." It is not permissible to delay prayer beyond its prescribed time except for a legitimate Shar'i excuse, such as sleep or forgetfulness. It is also not permissible to combine two prayers except for an excuse like hardship or difficulty, and not as a habit. It is not permissible to delay the 'Isha prayer until the time of Fajr.