Salaatut duha (chaast prayers)

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".