Isha (prayer)

Isha, alternatively also transliterated as Ishaa', and also known as Khoftan or Khooftaan, is one of the mandatory five daily Islamic prayers, and contains four cycles.

Ahadith mentioning virtues

reported that he heard Muhammad saying: "The one who offered Isha salat in a congregation, it was as if he remained in salat up to midnight, and he who offered the Fajr salat in a congregation, it was as if he remained in salat the whole night."
Abu Hurairah reported: The Messenger of Allah said, "The most difficult Salah for the Munafiqeen is Isha and Fajr. Had they known the rewards for them, they would have attended them even if they had to crawl on their knees."

Sunni Muslims

The time period within which the Isha prayer must be recited is the following:

Time begins: According to the Hanafi school, Isha begins when complete darkness has arrived and the white twilight in the sky has disappeared. According to the Maliki, Shafi'i and Hanbali schools, the time begins when the red thread has disappeared from the sky. These times can be approximated by using the sun as a measure. When the sun has descended 12 degrees below the horizon, it is approximately equivalent to the disappearance of the red from the sky. Islamic rationalist are of the opinion that the time of Isha prayer starts after civil dusk, when darkness starts spreading across the sky. For approximating when complete darkness begins, some astronomers argue that it occurs when the sun has descended 15 degrees below the horizon while others use the safer measure of 18 degrees.
Time ends: At the beginning of dawn when the time for Fajr prayer begins. However, it is frowned upon to delay the prayer without a legitimate reason past the first third of the night, and "night" in Islamic law means the time between the end of the Maghrib prayer and the start of the Fajr prayer. Islamic rationalist are of the opinion that the time of Isha prayer ends at nautical dusk, when the darkness spreads completely and the western horizon can no longer be observed through the naked eye. According to an opinion in the Maliki school, the prohibition is from delaying the prayer beyond the first half of the night, rather than just the first third.
Shia Muslims

The time period within which the Isha prayer must be recited is the following:
For Shia Muslims:

Time begins: once Maghrib has been recited and completed.
Time ends: at midnight, the midpoint between shafak and dawn.

However, it is very important to recite the prayer as soon as the time begins. Often Maghrib and Isha are offered together with a small gap of time in between.