Sahih Muslim
Sahih Muslim 719 c
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
Arabic hadith text
وَحَدَّثَنِي يَحْيَى بْنُ حَبِيبٍ الْحَارِثِيُّ، حَدَّثَنَا خَالِدُ بْنُ الْحَارِثِ، عَنْ سَعِيدٍ، حَدَّثَنَا قَتَادَةُ، أَنَّ مُعَاذَةَ الْعَدَوِيَّةَ، حَدَّثَتْهُمْ عَنْ عَائِشَةَ، قَالَتْ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلميُصَلِّي الضُّحَى أَرْبَعًا وَيَزِيدُ مَا شَاءَ اللَّهُ .
Translation
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.