Mu'aadhah Radiyallahu 'Anha says:
"I asked 'Aayeshah, 'Did Rasulullah Sallallahu'Alayhi Wasallam perform the salaatut duha (chaast prayers) ?" She replied: "Yes, he performed four rak'ahs (the least), and added to it the amount Allah Ta'aala willed".
English reference: Book 40, Hadith 272
Arabic reference: Book 41, Hadith 288