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". (IT)
English reference (IT): Book 40, Hadith 272 (IT)
Arabic reference (IT): Book 41, Hadith 288 (IT)