question #1 Sahih Muslim 1080e, Book 13, Hadith 8 : How is the beginning and end of Ramadan determined in Islamic tradition?

The month of Ramadan may consist of twenty-nine days. So do not fast till you have sighted it (the new moon) and do not break fast, till you have sighted it (the new moon of Shawwal), and if the sky is cloudy for you, then calculate.