Sahih Muslim Book 6, Hadith Number 2369.

Chapter : Fasting in Ramadan should necessarily be commenced with the sight of the new moon and finished with the sight of the new moon. If the weather is cloudy at the beginning or at the end, then complete thirty days.

‘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.