Narrated `Aisha radiyallahu `anhaa: Allah’s Messenger sallallahu `alayhi wa sallam used to pray four rak`at in Duha prayer and added whatever Allah wished for him. [Reported by Muslim]

Let it be known that Ishraaq, Salaat ul-Awwabin and Duha are different names that stand for identically the same forenoon prayer. The time of this prayer starts with the sunrise and goes on until one-fourth of the day. The least number of Rak`at of Duha prayer are two and the maximum is twelve. The preferable among them are four which is supported and approved by the Ahadith of the Prophet sallallahu `alayhi wa sallam. It may also be remembered that this prayer is a Sunnah of former Messengers as they used to offer it during their times. Hence this prayer is a Sunnah and mustahabb (desirable). As to the matter of this prayer being called a Bid`ah (innovation) by `Umar radiyallahu `anhu, it only alludes to the fact that if someone develops a habit of continually offering it, it may be regarded as an innovation, as the Prophet sallallahu `alayhi wa sallam did not always offer it.

 Narrated Zaid bin Arqam radiyallahu `anhu: Allah’s Messenger sallallahu `alayhi wa sallam, "The prayer of those who are penitent is offered when the young weaned camels feel the heat of the sun." [Reported by at-Tirmidhi]

This Hadith indicates that the best time of Duha prayer is just before noon.

Narrated Anas radiyallahu `anhu: Allah’s Messenger sallallahu `alayhi wa sallam said, "Whoever prays twelve rak`at of Duha, Allah will build a castle for him in Paradise." [Reported by at-Tirmidhi who graded it Gharib (unfamiliar, doubtful).]

Narrated `Aisha radiyallahu `anhaa: Allah’s Messenger sallallahu `alayhi wa sallam entered my house and prayed eight Rak`at of Duha prayer. [Reported by Ibn Hibban in his Sahih]. From Islaam.com