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