Answer
Praise be to Allah
Firstly
Salaat al-ishraaq is a two-rak‘ah prayer that is offered after the sun has risen and become high, for those who pray Fajr in congregation in the mosque, then sit in the place where they prayed, remembering Allah, may He be exalted, until they pray two rak‘ahs.
Concerning its virtue, the Prophet (blessings and peace of Allah be upon him) said: “Whoever prays Fajr in congregation, then sits remembering Allah until the sun rises, then prays two rak‘ahs, will have a reward like that of Hajj and ‘Umrah, complete, complete, complete.”
Narrated by at-Tirmidhi (586), from Anas ibn Maalik (may Allah be pleased with him).
There is a difference of scholarly opinion as to the soundness of this hadeeth. It was classed as da‘eef (weak) by some scholars and classed as hasan (good) by others. Among those who classed it as hasan was al-Albaani (may Allah have mercy on him) in Saheeh Sunan at-Tirmidhi.
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Shaykh Ibn Baaz (may Allah have mercy on him) was asked about it and said: This hadeeth has isnaads that are fine, thus it is regarded as coming under the heading of hasan because of corroborating evidence. This prayer is mustahabb after the sun has risen and reached the height of a spear, that is approximately fifteen or twenty minutes after the sun rises.
End quote from Fataawa ash-Shaykh Ibn Baaz, 25/171
Secondly
This prayer is mustahabb (encouraged), not obligatory, and it comes under the heading of Duha prayer, because the time for Duha prayer lasts from when the sun has risen high until just before the zenith (the time of Zuhr).
With regard to Duha prayer being Sunnah and mustahabb, it was narrated by al-Bukhaari (1178) and Muslim (721) that Abu Hurayrah (may Allah be pleased with him) said: My beloved (blessings and peace of Allah be upon him) advised me to do three things, which I will not forsake so long as I live: to fast three days of each month, to pray Duha, and not to sleep until I pray Witr.
READ MORE: Answers To Six Common Questions About Muslims
Shaykh Ibn ‘Uthaymeen (may Allah be pleased with him) was asked about Ishraaq and Duha prayer, and replied: The Sunnah of Ishraaq is (the same as) the Sunnah of Duha, but if it is done early, from the time when the sun has risen to the height of a spear, then it is Ishraaq prayer; if it is done at the end of its time or in the middle of its time, then it is Duha prayer. But it is (all) Duha prayer, because the scholars (may Allah be pleased with them) said that the time of Duha prayer lasts from when the sun has risen to the height of a spear until just before the zenith.
End quote from Liqa’ al-Baab al-Maftooh, 141/24.
Voluntary Prayers: The time of Duha prayer
Question
What is the proper time to pray “Israk” & “Chast” Prayer?.
Answer:
Praise be to Allaah.
Ishraaq (“ishrak”) prayer is Duha (“chast”) prayer offered at the beginning of its time. These are not two different prayers. It is called Ishraaq because it is done immediately after sunrise (shurooq) when the sun has risen to a certain height.
Shaykh Ibn Baaz said: Ishraaq prayer is Duha prayer done at the beginning of its time.
Majmoo’ Fataawa al-Shaykh Ibn Baaz, 11/401
The time of Duha prayer is from the time when the sun has risen to a certain height, until just before the time for Zuhr prayer.
Shaykh Ibn ‘Uthaymeen defined it as being from a quarter of an hour after the sun has risen until ten minutes before Zuhr prayer.
Al-Sharh al-Mumti’, 4/122
So all of this period is the time for Duha prayer.
It is better to pray it after the sun’s heat has become intense, because the Prophet (peace and blessings of Allaah be upon him) said: “It is preferable to offer Duha prayer when the sun’s heat has become so intense that even the young of the camels feel it.”
Narrated by Muslim, 748
Ibn Baaz: Majmoo’ Fataawa, 11/395
The scholars defined this as being when one quarter of the day has passed, i.e., halfway between sunrise and Zuhr prayer.
See al-Majmoo’ by al-Nawawi, 4/36; al-Mawsoo’ah al-Fiqhiyyah, 27/224 .