Shama-il Tirmidhi Chapter 042, Hadith Number 011 (291).

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".

Commentary
At certain times it was the practice of Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam not to give importance to fixing certain days. At times he fixed certain days. For example, fasting on the first three days of the month, or sometimes on the last three days of the month, or in some months on Saturday, Sunday and Monday, and in another month on Tuesdays, Wednesdays and Thursdays. Therefore different ahaadith have been narrated on this subject, and Sayyiditina 'Aayeshah, Radhiyallahu 'Anha refused to specify a certain day.