"I heard Mu'adhah saying to Aishah: 'Did the Messenger of Allah fast three days of every month?' She replied in the affirmative. So she said: 'Which of them would he fast?' She said: 'Which of them he would fast was not noticeable.'"
Reference: Jami` at-Tirmidhi 763 In-book : Book 8, Hadith 82 English translation : Vol. 2, Book 3, Hadith 763
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