Sahih Muslim
Sahih Muslim 719 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
حَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، وَابْنُ، بَشَّارٍ قَالاَ حَدَّثَنَا مُحَمَّدُ بْنُ جَعْفَرٍ، حَدَّثَنَا شُعْبَةُ، عَنْ يَزِيدَ، بِهَذَا الإِسْنَادِ . مِثْلَهُ وَقَالَ يَزِيدُ مَا شَاءَ اللَّهُ
Translation
A hadith like this has been reported by the same chain of transmitters, but with this alteration that the transmitter said
" As Allah pleased."