كتاب صلاة المسافرين وقصرها
6  
The Book of Prayer - Travellers
(13)
Chapter: 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
(13)
باب اسْتِحْبَابِ صَلاَةِ الضُّحَى وَأَنَّ أَقَلَّهَا رَكْعَتَانِ وَأَكْمَلَهَا ثَمَانِ رَكَعَاتٍ وَأَوْسَطَهَا أَرْبَعُ رَكَعَاتٍ أَوْ سِتٌّ وَالْحَثِّ عَلَى الْمُحَافَظَةِ عَلَيْهَا
Sahih Muslim 336 f

Abu Murra, the freed slave of Umm Hani, daughter of Abu Talib, reported Umm Hani to be saying:

I went to the Messenger of Allah (ﷺ) on the day of the Conquest of Mecca and found him taking a bath, and Fatimah, his daughter, had provided him privacy with the help of a cloth. I gave him salutation and he said: Who is she? I said: It is Umm Hani, daughter of Abu Talib. He (the Holy Prophet) said: Greeting for Umm Hani. When he had completed the bath, he stood up and observed eight rak'ahs wrapped up in one cloth. When he turned back (after the prayer), I said to him: Messenger of Allah, the son of my mother 'Ali b. Abu Talib is going to kill a person, Fulan b. Hubaira whom I have given protection. Upon this the Messenger of Allah (ﷺ) said: We too have given protection whom you have given protection, O Umm Hani. Umm Hani said: It was the forenoon (prayer).
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنْ أَبِي النَّضْرِ، أَنَّ أَبَا مُرَّةَ، مَوْلَى أُمِّ هَانِئٍ بِنْتِ أَبِي طَالِبٍ أَخْبَرَهُ أَنَّهُ، سَمِعَ أُمَّ هَانِئٍ بِنْتَ أَبِي طَالِبٍ، تَقُولُ ذَهَبْتُ إِلَى رَسُولِ اللَّهِ صلى الله عليه وسلم عَامَ الْفَتْحِ فَوَجَدْتُهُ يَغْتَسِلُ وَفَاطِمَةُ ابْنَتُهُ تَسْتُرُهُ بِثَوْبٍ - قَالَتْ - فَسَلَّمْتُ فَقَالَ ‏"‏ مَنْ هَذِهِ ‏"‏ ‏.‏ قُلْتُ أُمُّ هَانِئٍ بِنْتُ أَبِي طَالِبٍ ‏.‏ قَالَ ‏"‏ مَرْحَبًا بِأُمِّ هَانِئٍ ‏"‏ ‏.‏ فَلَمَّا فَرَغَ مِنْ غُسْلِهِ قَامَ فَصَلَّى ثَمَانِيَ رَكَعَاتٍ مُلْتَحِفًا فِي ثَوْبٍ وَاحِدٍ ‏.‏ فَلَمَّا انْصَرَفَ قُلْتُ يَا رَسُولَ اللَّهِ زَعَمَ ابْنُ أُمِّي عَلِيُّ بْنُ أَبِي طَالِبٍ أَنَّهُ قَاتِلٌ رَجُلاً أَجَرْتُهُ فُلاَنُ بْنُ هُبَيْرَةَ ‏.‏ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ قَدْ أَجَرْنَا مَنْ أَجَرْتِ يَا أُمَّ هَانِئٍ ‏"‏ ‏.‏ قَالَتْ أُمُّ هَانِئٍ وَذَلِكَ ضُحًى ‏.‏
Reference : Sahih Muslim 336f
In-book reference : Book 6, Hadith 99
USC-MSA web (English) reference : Book 4, Hadith 1555
  (deprecated numbering scheme)