The Messenger of Allah ﷺ used to fast three days of every month.She said, "I asked her, 'Which days of the month did he fast?'"She replied, "He did not mind which days of the month they were."
حدثنا محمد بن جعفر حدثنا شعبة عن يزيد الرشك عن معاذة
كان رسول الله ﷺ يصوم ثلاثة أيام من كل شهر قالت فقلت من أيه كان فقالت لم يكن يبالي من أيه كان
Its isnad is sahih. Muslim narrated it, 2/818, no. 1160, Abu Dawud, 2/328, no. 2453, and Tirmidhi, 3/126, no. 763, saying, hasan sahih, and Ibn Majah, 1/545, no. 1709, all of them in the chapters on fasting.