It was narrated form Ibn 'Umar that the Prophet said: D not fast until you see it , and do not stop fasting until you see it, and if it is obscured from you (too cloudy), then work it out.
It was narrated that Abu Hurairah said: The Messenger of Allah mentioned the crescent and said: 'When you see it, fast and when you see it, stop fasting, and if it is obscured from you (too cloudy), then count thirty days. '