Book 6, Number 2364:

Ibn Umar reported that Allah's Messenger (may peace be upon him) made a mention of Ramadan and he with the gesture of his hand said: The month is thus and thus. (He then withdrew his thumb at the third time). He then said: Fast when you see it, and break your fast when you see it, and if the weather is cloudy calculate it (the months of Sha'ban and Shawwal) as thirty days.