Sahih Muslim
Sahih Muslim 1162 e
It is recommended to fast three days of every month, and to fast on the days of 'Arafah and 'Ashura', and to fast on Mondays and Thursdays
Arabic hadith text
وَحَدَّثَنِي زُهَيْرُ بْنُ حَرْبٍ، حَدَّثَنَا عَبْدُ الرَّحْمَنِ بْنُ مَهْدِيٍّ، حَدَّثَنَا مَهْدِيُّ بْنُ مَيْمُونٍ، عَنْ غَيْلاَنَ، عَنْ عَبْدِ اللَّهِ بْنِ مَعْبَدٍ الزِّمَّانِيِّ، عَنْ أَبِي قَتَادَةَ الأَنْصَارِيِّ، رضى الله عنه أَنَّرَسُولَ اللَّهِ صلى الله عليه وسلم سُئِلَ عَنْ صَوْمِ الاِثْنَيْنِ فَقَالَ "فِيهِ وُلِدْتُ وَفِيهِ أُنْزِلَ عَلَىَّ "
Translation
Abu Qatada Ansari (Allah be pleased with him) reported that Allah's Massenger (ﷺ) was asked about fasting on Monday, whereupon he said
It is (the day) when I was born and revelation was sent down to me.