question #3 Sahih Muslim 1080f, Book 13, Hadith 9 : How many days can the month of Ramadan consist of according to Islamic teachings?

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).