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 d

Abd al-Rahman b. Abu Laila reported:

No one has ever narrated to me that he saw the Messenger of Allah (ﷺ) observing the forenoon prayer, except Umm Hani. She, however, narrated that the Messenger of Allah (ﷺ) entered her house on the day of the Conquest of Mecca and prayed eight rak'ahs (adding): I never saw a shorter prayer than it except that he performed the bowing and prostration completely. But (one of the narrators) Ibn Bashshar in his narration made no mention of the word:" Never".
وَحَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، وَابْنُ، بَشَّارٍ قَالاَ حَدَّثَنَا مُحَمَّدُ بْنُ جَعْفَرٍ، حَدَّثَنَا شُعْبَةُ، عَنْ عَمْرِو بْنِ مُرَّةَ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ أَبِي لَيْلَى، قَالَ مَا أَخْبَرَنِي أَحَدٌ، أَنَّهُ رَأَى النَّبِيَّ صلى الله عليه وسلم يُصَلِّي الضُّحَى إِلاَّ أُمُّ هَانِئٍ فَإِنَّهَا حَدَّثَتْ أَنَّ النَّبِيَّ صلى الله عليه وسلم دَخَلَ بَيْتَهَا يَوْمَ فَتْحِ مَكَّةَ فَصَلَّى ثَمَانِيَ رَكَعَاتٍ مَا رَأَيْتُهُ صَلَّى صَلاَةً قَطُّ أَخَفَّ مِنْهَا غَيْرَ أَنَّهُ كَانَ يُتِمُّ الرُّكُوعَ وَالسُّجُودَ ‏.‏ وَلَمْ يَذْكُرِ ابْنُ بَشَّارٍ فِي حَدِيثِهِ قَوْلَهُ قَطُّ ‏.‏
Reference : Sahih Muslim 336d
In-book reference : Book 6, Hadith 97
USC-MSA web (English) reference : Book 4, Hadith 1553
  (deprecated numbering scheme)