Duha/Ishraq/Chasht Prayer (Hadith No. 906)


Volume 2, Book 20, Number 207:
Narrated Ibn Abu Laila:
Only Um Hani told us that she had seen the Prophet (p.b.u.h) offering the Duha (forenoon prayer). She said, "On the day of the conquest of Mecca, the Prophet took a bath in my house and offered eight Rakat. I never saw him praying such a light prayer but he performed perfect prostration and bowing. Narrated 'Abdullah bin amir that his father had told him that he had seen the Prophet (p.b.u.h) praying Nawafil at night on the back of his Mount on a journey, facing whatever direction it took.

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.