The Messenger of Allah ﷺ used to fast three days each month, Monday, Thursday, and the Monday of the following week.
حدثنا موسى بن إسمعيل حدثنا حماد عن عاصم بن بهدلة عن سواء الخزاعي
كان رسول الله ﷺ يصوم ثلاثة أيام من الشهر الاثنين والخميس والاثنين من الجمعة الأخرى
[Its chain of transmission is hasan] Nasai narrated it in the Book of Fasting, chapter on the fasting of the Prophet ﷺ, may my father and mother be sacrificed for him, and the mention of the differences among the transmitters of the report on that matter, hadith no. 2368, from the hadith of Hammad bin Salama, through this route. Sawa Khuzai was deemed trustworthy by Ibn Hibban, and Ibn Khuzayma graded his hadith sahih, so his narrations are hasan.