(13) باب اسْتِحْبَابِ صَلاَةِ الضُّحَى وَأَنَّ أَقَلَّهَا رَكْعَتَانِ وَأَكْمَلَهَا ثَمَانِ رَكَعَاتٍ وَأَوْسَطَهَا أَرْبَعُ رَكَعَاتٍ أَوْ سِتٌّ وَالْحَثِّ عَلَى الْمُحَافَظَةِ عَلَيْهَا
Abu Murra, the freed slave of Umm Hani, narrated on the authority of Abu Darda':
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.
Reference: Sahih Muslim 722
In-book reference: Book 6, Hadith 105
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1561