Sahih Muslim Book 6, Hadith Number 2605.

Chapter : Excellence of observing fast for three days during every month, and fasting on the day of ‘Arafa, and ‘Ashura and Monday and Thursday.

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.