Narrated 'A'ishah radhiallahu 'anha: 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 Ishraq, Salat 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 radhiallahu '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 Zayd ibn Arqam radhiallahu 'anhu: Allah's Messenger sallallahu 'alayhi wa sallam said:
"The prayer of those who are penitent is offered when the young weaned camels feel the heat of the sun."This hadith indicates that the best time of Duha prayer is just before noon.
Narrated Anas radhiallahu '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."Narrated 'A'ishah radhiallahu 'anha: 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]
Excerpted from Al-Hafidh Ibn Hajar al-'Asqalani's Bulugh al-Maram, with notes from As-San'ani's commentary Subul as-Salam