This hadeeth comes under the following 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 (Allaah be pleased with them) reported that the Prophet of Allaah (sallAllaahu alayhi wa sallam) 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 Allaah) said: When you break it, then observe fast for two days.