Sahih Muslim | Book 6 | Hadith Number 2369

Narrated by 'Abdullah b. Umar (Allah be pleased with both of them)
'Abdullah b. Umar (Allah be pleased with both of them) reported Allah's Messenger (may peace be upon him) as saying: When you see the new moon, observe fast, and when you see it (again) then break it, and if the sky is cloudy for you, then calculate it.