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 719 b

A hadith like this has been reported by the same chain of transmitters, but with this alteration that the transmitter said:

" As Allah pleased."
حَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، وَابْنُ، بَشَّارٍ قَالاَ حَدَّثَنَا مُحَمَّدُ بْنُ جَعْفَرٍ، حَدَّثَنَا شُعْبَةُ، عَنْ يَزِيدَ، بِهَذَا الإِسْنَادِ ‏.‏ مِثْلَهُ وَقَالَ يَزِيدُ مَا شَاءَ اللَّهُ ‏.‏
Reference : Sahih Muslim 719b
In-book reference : Book 6, Hadith 94
USC-MSA web (English) reference : Book 4, Hadith 1550
  (deprecated numbering scheme)