Sahih Muslim | Book 6 | Hadith Number 2378

Narrated by Abu Huraira
Abu Huraira reported Allah's Messenger (may peace be upon him) as saying: Whenever you sight the new moon (of the month of Ramadan) observe fast. and when you sight it (the new moon of Shawwal) break it, and if the sky is cloudy for you, then observe fast for thirty days.