Whenever you sight the new moon (of the month of Ramadan) observe fast. and when you sight it (the new moon of Shawwal) break it, and if the sky is cloudy for you, then observe fast for thirty days.
Sahih Muslim Book 13, Hadith 20
← Previous Hadith This night is the midnight (of the month). Upon this he said to him: How do you know that it is the midnight (of the month), for I heard the Messenger of Allah (ﷺ) as saying: The month is thus and thus (and he pointed with his ten fingers twice) and thus (i. e. at the third time he pointed with all his fingers but withdrew or folded his thumb)?

Sahih Muslim Book 13, Hadith 19 Next Hadith → Observe fast on sighting it (the new moon) and break (fast) on sighting it (the new moon), but if the sky is cloudy for you, then complete the number (of thirty).

Sahih Muslim Book 13, Hadith 21