Timing of ‘Isha prayer

This question is in reference to the timings of Isha Salat. There are various opinions regarding, when the time for offering Isha prayer expires. Some say that one can pray Isha till the Fajr Azaan is called, while others say that it expires at the time of "Tahajjud prayers". And finally some believe, that we are supposed to take the number of hours between Isha Azaan and the Fajr Azaan, and divide them into half, in order to estimate the last time for Isha prayer.
I know that prayers should be offered promptly and on time. Delaying a prayer without reason is not encouraged, but still it would be very beneficial to know the exact opinion on this issue.

Praise be to Allaah.

‘Isha prayer must be performed before midnight, and it is not permissible to delay it until midnight, because the Prophet (peace and blessings of Allaah be upon him) said: “The time of ‘Isha’ is until midnight” (narrated by Muslim, al-Masaajid wa Mawaadi’ al-Salaah, 964). So you have to pray it before midnight, based on the length of the night, because the night may be longer or shorter, so the guideline is how many hours the night lasts. If the night is ten hours long, then it is not permissible to delay it until the end of the fifth hour. The best way it to pray it in the first third of the night. If a person prays it at the beginning of the time for ‘Isha, that is OK, but if he delays it a little while, that is preferable, because the Messenger (peace and blessings of Allaah be upon him) used to recommend delaying ‘Isha’ prayer for a little while. But if someone prays it at the beginning of its time, after the twilight – the reddish afterglow along the horizon – has vanished, there is nothing wrong with that. And Allaah knows best.

Majmoo’ah Fataawa al-Shaykh ‘Abd al-‘Azeez ibn Baaz, 10/386