Abu Qatadah reported: The Prophet, peace and blessings be upon him, was asked about fasting on Monday. The Prophet said, “That is the day I was born and the day I was sent with revelation. Fasting three days of every month and Ramadan after Ramadan is like fasting for a lifetime.”
Source: Ṣaḥīḥ Muslim 1162
Grade: Sahih (authentic) according to Muslim
عَنْ أَبِي قَتَادَةَ عَنْ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ وَسُئِلَ عَنْ صَوْمِ يَوْمِ الاِثْنَيْنِ فَقَالَ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ ذَاكَ يَوْمٌ وُلِدْتُ فِيهِ وَيَوْمٌ بُعِثْتُ أَوْ أُنْزِلَ عَلَىَّ فِيهِ صَوْمُ ثَلاَثَةٍ مِنْ كُلِّ شَهْرٍ وَرَمَضَانَ إِلَى رَمَضَانَ صَوْمُ الدَّهْرِ
1162 صحيح مسلم كتاب الصيام باب استحباب صيام ثلاثة أيام من كل شهر وصوم يوم عرفة وعاشوراء والاثنين والخميس