(13) باب اسْتِحْبَابِ صَلاَةِ الضُّحَى وَأَنَّ أَقَلَّهَا رَكْعَتَانِ وَأَكْمَلَهَا ثَمَانِ رَكَعَاتٍ وَأَوْسَطَهَا أَرْبَعُ رَكَعَاتٍ أَوْ سِتٌّ وَالْحَثِّ عَلَى الْمُحَافَظَةِ عَلَيْهَا
Abu Huraira reported. My friend (the Holy Prophet, may peace be upon him) has instructed me to do three things:
three fasts during every month, two rak'ahs of the forenoon prayer, and observing Witr prayer before going to bed.
Reference: Sahih Muslim 721 a
In-book reference: Book 6, Hadith 102
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1558