صَحِيحُ مُسْلِمٍ
Sahih Muslim 1161 a
The Book of Fasting
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
وَحَدَّثَنِي عَبْدُ اللَّهِ بْنُ مُحَمَّدِ بْنِ أَسْمَاءَ الضُّبَعِيُّ، حَدَّثَنَا مَهْدِيٌّ، - وَهُوَ ابْنُ مَيْمُونٍ - حَدَّثَنَا غَيْلاَنُ بْنُ جَرِيرٍ، عَنْ مُطَرِّفٍ، عَنْ عِمْرَانَ بْنِ حُصَيْنٍ، - رضى الله عنهما - أَنَّصلى الله عليه وسلم قَالَ لَهُ أَوْ قَالَ لِرَجُلٍ وَهُوَ يَسْمَعُ " يَا فُلاَنُ أَصُمْتَ مِنْ سُرَّةِ هَذَا الشَّهْرِ " . قَالَ لاَ . قَالَ " فَإِذَا أَفْطَرْتَ فَصُمْ يَوْمَيْنِ " .
'Imran b. Husain (Allah be pleased with them) reported that the Messenger of Allah (ﷺ) said to him (or he said to another person and he was listening to it): O, so and so, did you observe fast in the middle of the month? He said: No. Thereupon he (the Messenger of Allah) said: When you break it, then observe fast for two days.
Sahih Muslim, 1161 aSahihصحيح