From Sahih Muslim (Hadith 2499):
Ibn Umar reported that Allah's Messenger (ﷺ) 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
Reference: Muslim 2499