Hadith #1003
English (English)It is in Sahih Muslim from Sayyiduna Abu Qatadah al-Ansari (RadiyAllahu Anhu) that Rasoolullah ﷺ was asked about his reason for fasting on a Monday, so He ﷺ said: It is (the day) on which I was BORN, and it is (the day) on which Revelation (Wahi) descended upon me.
Reference: صحيح مسلم، كتاب الصيام، باب استحباب صيام ثلاثة أيام۔۔۔الخ، الحدیث 1162