The Hadith you're referring to is likely the one where the Prophet Muhammad (peace be upon him) said, "Fast three days a month, as it is equivalent to fasting the entire year." (Narrated by Bukhari and Muslim).
This Hadith highlights the importance and reward of observing a 3-day fast, which is considered a form of spiritual purification and a means of seeking closeness to Allah. The exact days of the 3-day fast are not specified in the Hadith, but it's commonly practiced on the 13th, 14th, and 15th of each lunar month (known as the "White Days").