Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
You should not put off praying Isha to pray it close to Fajr, if you have a choice; however, if you could not pray earlier because of circumstances beyond your control then you may pray it before the arrival of Fajr.
Responding to your question, Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
Best time to offer Isha prayer
Ideally, one should pray Isha before midnight. It is important for us to be diligent in prayer and not put it off unnecessarily when the time arrives, although if someone is praying Isha in congregation, it is best to delay it up to one third of the night; but it is not good to postpone it later than midnight.
This is what we can infer from the traditions from the Prophet (peace be upon him). The Prophet (peace be upon him) said, “The (preferred) time of Isha prayer extends to the middle part of the night.”
There are other traditions in which the Prophet exhorts us to delay Isha: “If it wouldn’t have caused hardship on my ummah, I would have ordered them to delay Isha.” However, no where he told us to delay it past midnight.
Delaying Isha prayer
In light of the traditions mentioned above and other valid inferences, scholars are of the view that ideally one should not delay Isha farther than midnight. However, if a person could not pray before midnight, he could still pray it before dawn–as the permissible time extends up to that time.
We read in the Mughni of Ibn Qudamah (who is of Hanbali school): “Ideally, one should not delay Isha farther than one third of the night, if he did delay it until midnight it is fine; after midnight, it is the time of darurah (i.e. the time for those in extreme circumstances or valid excuses).”
Imam An-Nawawi (who belongs to Shafi`i school) says, “The ideal time of Isha extends up to one third of the night; beyond that is the permissible time; which extends to the arrival of dawn.”
The Hanafi view is almost the same as mentioned above. Thus Imam At-Tahawi (who is affiliated to Hanafi school) states: “The time of Isha can be divided into three: The ideal or the most preferred time extends to one third of the night; after which is the less preferable time which extends to midnight, still less preferable is the time past midnight until the arrival of dawn.”
To conclude, you should not put off praying Isha to pray it close to Fajr, if you have a choice; however, if you could not pray earlier because of circumstances beyond your control then you may pray it before the arrival of Fajr.
Almighty Allah knows best.