Until when may one establish ‘isha prayer?
The time of ‘isha prayer starts after that of maghrib prayer ends, and lasts until “ fajr” time (start of dawn’s breaking) (Tahawi, Sharh ma‘ani al-asar, I, 159; Kasani, Bada’i, I, 124; Marghinani, al-Hidayah, I, 258; Zaylai, Tabyin, I, 81). One may establish ‘isha prayer any time within this period. However, some scholars said that, like every fardh prayer, it is more virtuous to establish ‘isha prayer within the first part of its time, as advised by the Prophet (saw). On the contrary, there are some scholars who say that it is more appropriate to establish ‘isha prayer in later parts of night, based on some other narratives (Ibn Qudamah, al-Mughni, II, 28).
According to the Shafi’i school, the time of ‘isha prayer starts when the glow in the western horizon disappears, and lasts until the dawn breaks. However, the time of ‘isha prayer is divided into four in itself according to the Shafi’i school as: “virtuous”, “optional”, “permissible”, and “excuse”. Virtuous time is to perform it at the beginning of the time. Optional time is the first one third of the night. The time after this until dawn is permissible time. Although it is permissible to establish ‘isha prayer in this time, it is considered as makruh. Excuse time is the time of maghrib prayer when ‘isha will be established by means of jam al-taqdim (Nawavi, al-Majmu’, III, 31).
Source: Presidency Of Religious Affairs The Turkey, High Board of Religious Affairs FATWAS