Sahih Muslim | Book 6 | Hadith Number 2363

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 in connection with Ramadan: Do not fast till you see the new moon, and do not break fast till you see it; but if the weather is cloudy calculate about it.