Ishraq Prayer: Sunnah or Obligatory?
Question

Is it obligatory to offer Ishraq prayer (prayer following sunrise)?
Praise be to Allah.Firstly: 
What is Ishraq prayer?
Ishraq prayer 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. 
Virtue of Ishraq prayer
Concerning the virtue of ishraq prayer, 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 Malik (may Allah be pleased with him) 
Degree of hadith on Ishraq prayer
There is a difference of scholarly opinion as to the soundness of this hadith. It was classed as da‘if (weak) by some scholars and classed as hasan (good) by others. Among those who classed it as hasan was al-Albani (may Allah have mercy on him) in Sahih Sunan at-Tirmidhi. 
Shaykh Ibn Baz (may Allah have mercy on him) was asked about it and said: 
“This hadith has isnads 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.” (Fatawa ash-Shaykh Ibn Baz, 25/171)
Ishraq prayer: sunnah or obligatory?
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). 
Is Duha prayer recommended?
With regard to Duha prayer being Sunnah and mustahabb, it was narrated by al-Bukhari (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. 
Ishraq prayer and Duha prayer: The Same?
Shaykh Ibn ‘Uthaymin (may Allah be pleased with him) was asked about Ishraq and Duha prayer, and replied: 
“The Sunnah of Ishraq 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 Ishraq 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.” (Liqa al-Bab al-Maftuh, 141/24)  
And Allah knows best.