Sahih Muslim 722
Sahih Muslim · Book 6: 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
Abu Murra, the freed slave of Umm Hani, narrated on the authority of Abu Darda':
وَحَدَّثَنِي هَارُونُ بْنُ عَبْدِ اللَّهِ ، وَمُحَمَّدُ بْنُ رَافِعٍ ، قَالاَ حَدَّثَنَا ابْنُ أَبِي فُدَيْكٍ ، عَنِ الضَّحَّاكِ بْنِ عُثْمَانَ ، عَنْ إِبْرَاهِيمَ بْنِ عَبْدِ اللَّهِ بْنِ حُنَيْنٍ ، عَنْ أَبِي مُرَّةَ ، مَوْلَى أُمِّ هَانِئٍ عَنْ أَبِي الدَّرْدَاءِ ، قَالَ أَوْصَانِي حَبِيبِي صلى الله عليه وسلم بِثَلاَثٍ لَنْ أَدَعَهُنَّ مَا عِشْتُ بِصِيَامِ ثَلاَثَةِ أَيَّامٍ مِنْ كُلِّ شَهْرٍ وَصَلاَةِ الضُّحَى وَبِأَنْ لاَ أَنَامَ حَتَّى أُوتِرَ .
My Friend (ﷺ) instructed me in three (acts), and I would never abandon them as long as I live. (And these three things are): Three fasts during every month, the forenoon prayer, and this that I should not sleep till I have observed the Witr prayer.