بُلُوغُ الْمَرَامِ
Bulugh al-Maram 392
The Book of Prayer
وَعَنْ عَائِشَةَ -رَضِيَ اَللَّهُ عَنْهَا- قَالَتْ: { كَانَ رَسُولُ اَللَّهِ - صلى الله عليه وسلم -يُصَلِّي اَلضُّحَى أَرْبَعًا, وَيَزِيدُ مَا شَاءَ اَللَّهُ } رَوَاهُ مُسْلِمٌ . [1] .
Narrated 'Aishah (RA): Allah's Messenger (ﷺ) used to pray four Rak'at in Duha prayer and increase (the number of Rak'at) as much as Allah willed." [Reported by Muslim].
Sunnah.com reference: Book 2, Hadith 296