سم الله الرحمن الرحيم
Bismillā hir Rahmā nir Rahīm
In the name of Allah, Most Gracious, Most Merciful
There are three opinions on when Isha Prayer expires. Whatever opinion you adopt be consistent in the application. Insha Allah
Muslims have been directed by Prophet Muhammad (SAWS) to perform salat at the time it due. But there are always exceptions/
Sahih Bukhari Volume 1, Book 11, Number 627:
Narrated Malik bin Huwairith:
Prophet said (to two persons), "Whenever the prayer time becomes due, you should pronounce Adhan and then Iqama and the older of you should lead the prayer."
1. Isha Salat expires until the Adhan of Fajr.
2. Isha Salat expires until time of Tahajjud Salat
3. Isha Salat expires by calulating the number of hours bewteen Isha Adhan and Fajr Adhan and dividing the number in half to estimate the last time Isha Salat can be perform.
Sahih Bukhari Volume 9, Book 90, Number 345:
Narrated 'Ata:
One night the Prophet delayed the Isha' prayer whereupon 'Umar went to him and said, "The prayer, O Allah's Apostle! The women and children had slept." The Prophet came out with water dropping from his head, and said, "Were I not afraid that it would be hard for my followers (or for the people), I would order them to pray Isha prayer at this time." (Various versions of this Hadith are given by the narrators with slight differences in expression but not in content).