Sahih Muslim Book 6, Hadith Number 2363.

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.

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 breakfast till you see it; but if the weather is cloudy calculate about it.