Sahih Muslim Book 6, Hadith Number 2601.

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

‘Imran b. Husain (Allah be pleased with them) reported that the Apostle of Allah (may peace be upon him) 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.