كتاب الصيام
13
The Book of Fasting
(36)
Chapter: 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
(36)
باب اسْتِحْبَابِ صِيَامِ ثَلَاثَةِ أَيَّامٍ مِنْ كُلِّ شَهْرٍ وَصَوْمِ يَوْمِ عَرَفَةَ وَعَاشُورَاءَ وَالِاثْنَيْنِ وَالْخَمِيسِ
Sahih Muslim 1162 d

This hadith has been narrated by Ghailan b. Jarir with the same chain of transmitters, but with one variation, that there has been made mention of Monday and not of Thursday.

وَحَدَّثَنِي أَحْمَدُ بْنُ سَعِيدٍ الدَّارِمِيُّ، حَدَّثَنَا حَبَّانُ بْنُ هِلاَلٍ، حَدَّثَنَا أَبَانُ الْعَطَّارُ، حَدَّثَنَا غَيْلاَنُ بْنُ جَرِيرٍ، فِي هَذَا الإِسْنَادِ بِمِثْلِ حَدِيثِ شُعْبَةَ غَيْرَ أَنَّهُ ذَكَرَ فِيهِ الاِثْنَيْنِ وَلَمْ يَذْكُرِ الْخَمِيسَ ‏.‏
Reference : Sahih Muslim 1162 d
In-book reference : Book 13, Hadith 255
USC-MSA web (English) reference : Book 6, Hadith 2605
  (deprecated numbering scheme)