Sahih Muslim
Sahih Muslim 721 b
It is recommended to pray Duha, the least of which is two rak`ah, the best of which is eight, and the average of which is four or six, and encouragement to do so regularly
Arabic hadith text
وَحَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، وَابْنُ، بَشَّارٍ قَالاَ حَدَّثَنَا مُحَمَّدُ بْنُ جَعْفَرٍ، حَدَّثَنَا شُعْبَةُ، عَنْ عَبَّاسٍ الْجُرَيْرِيِّ، وَأَبِي، شِمْرٍ الضُّبَعِيِّ قَالاَ سَمِعْنَا أَبَا عُثْمَانَ النَّهْدِيَّ، يُحَدِّثُ عَنْ أَبِي هُرَيْرَةَ، عَنِ النَّبِيِّ صلى الله عليه وسلم . بِمِثْلِهِ .
Translation
A hadith like this has been narrated by Abu Huraira by another chain of transmitters.