صَحِيحُ مُسْلِمٍ
Sahih Muslim 721 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 أَبُو التَّيَّاحِ، حَدَّثَنِي أَبُو عُثْمَانَ النَّهْدِيُّ، عَنْ أَبِي هُرَيْرَةَ، قَالَ أَوْصَانِي خَلِيلِي صلى الله عليه وسلم بِثَلاَثٍ بِصِيَامِ ثَلاَثَةِ أَيَّامٍ مِنْ كُلِّ شَهْرٍ وَرَكْعَتَىِ الضُّحَى وَأَنْ أُوتِرَ قَبْلَ أَنْ أَرْقُدَ .
Abu Huraira reported. My friend (the Prophet, ﷺ) has instructed me to do three things: three fasts during every month, two rak'ahs of the forenoon prayer, and observing Witr prayer before going to bed.
Sahih Muslim, 721 aSahihصحيح