صَحِيحُ مُسْلِمٍ
Sahih Muslim 719 a
The Book of Prayer - Travellers
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
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 .
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.
Sahih Muslim, 719 aSahihصحيح