From Sahih Muslim (Hadith 2503):
Abdullah b. 'Umar (Allah be pleased with both of them) reported Allah's Messenger (ﷺ) as saying:The month (of Ramadan) may consist of twenty nine days; so when you see the new moon observe fast and when you see (the new moon again at the commencement of the month of Shawwal) then break It, and if the sky is cloudy for you, then calculate it (and complete thirty days)
Reference: Muslim 2503