Sahih Muslim Book 6, Hadith Number 2365.

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.

This Hadith is narrated on the authority of ‘Ubaidullah with the same chain of transmitters, and he said: If (the sky) is cloudy for you, then calculate thirty days (for the month of Ramadan).