Sahih Muslim
Sahih Muslim 719 a
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 شَيْبَانُ بْنُ فَرُّوخَ، حَدَّثَنَا عَبْدُ الْوَارِثِ، حَدَّثَنَا يَزِيدُ، - يَعْنِي الرِّشْكَ - حَدَّثَتْنِي مُعَاذَةُ، أَنَّهَا سَأَلَتْ عَائِشَةَ - رضى الله عنها -كَمْ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يُصَلِّي صَلاَةَ الضُّحَى قَالَتْ أَرْبَعَ رَكَعَاتٍ وَيَزِيدُ مَا شَاءَ .
Translation
Mu'adha asked 'A'isha (Allah be pleased with her) how many rak'ahs Allah's Messenger (ﷺ) prayed at the forenoon prayer. She replied
Four rak'ahs, but sometimes more as he pleased.