(13) باب اسْتِحْبَابِ صَلاَةِ الضُّحَى وَأَنَّ أَقَلَّهَا رَكْعَتَانِ وَأَكْمَلَهَا ثَمَانِ رَكَعَاتٍ وَأَوْسَطَهَا أَرْبَعُ رَكَعَاتٍ أَوْ سِتٌّ وَالْحَثِّ عَلَى الْمُحَافَظَةِ عَلَيْهَا
Mua'ada 'Adawiyya reported 'A'isha as saying:
The Messenger of Allah (ﷺ) used to observe four rak'ahs in the forenoon prayer and he sometimes observed more as Allah pleased.
Reference: Sahih Muslim 719 c
In-book reference: Book 6, Hadith 95
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1551