What did Prophet Muhammad (peace be upon him) say about fasting during Ramadan if the sky is overcast?
 Narrated Ibn `Umar:                     I heard Allah's Apostle saying, "When you see the crescent (of the month of Ramadan), start fasting, and when you see the crescent (of the month of Shawwal), stop fasting; and if the sky is overcast (and you can't see it) then regard the month of Ramadan as of 30 days."