Sahih Muslim | Book 6 | Hadith Number 2367

Narrated by Ibn'Umar (Allah be pleased with-both of them)
Ibn'Umar (Allah be pleased with-both of them) reported Allah's Messenger (may peace be upon him) as saying: The month of Ramadan may consist of twenty-nine days. So do not fast till you have sighted it (the new moon) and do not break fast, till you have sighted it (the new moon of Shawwal), and if the sky is cloudy for you, then calculate.