question #1 Sahih Muslim 1080h, Book 13, Hadith 11 : How does the sighting of the new moon determine the beginning and end of the fasting month in Islam?

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