Mu'aadhah Radiyallahu 'Anha says. "I inquired from 'Aayeshah Radiyallahu 'Anha, did Rasulullah Sallallahu 'Alayhi Wasallam fast for three days of every month?" She replied:
"Yes". I then asked: "On which days of the month did he fast". She replied. "He did not fast on specific days, but whenever suitable".
English reference: Book 42, Hadith 291
Arabic reference: Book 43, Hadith 308