ENOfficial
Mu'adha said she asked ‘A’isha how many rak'as God’s Messenger prayed at the forenoon prayer, and she replied, “Four rak'as, but sometimes more, as God pleased.” Muslim transmitted it.
Source: sunnah.com
Other translations
Arabic
وَعَن معَاذَة قَالَتْ: سَأَلْتُ عَائِشَةَ: كَمْ كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يُصَلِّي صَلَاةَ الضُّحَى؟ قَالَتْ: أَرْبَعَ رَكَعَاتٍ وَيَزِيدُ مَا شَاءَ اللَّهُ. رَوَاهُ مُسلم
sunnah.com