The Prophet (peace and blessings be upon him) said: “When you sight it, fast; and when you sight it, end the fast. If it is obscured from you, then calculate its duration.” Agreed upon1.
Another wording states: “Then reckon it2 as thirty days.” Another states: “Then complete the number of days of Shaʿbān as thirty.” Narrated by al-Bukhārī3.
243—Fasting begins when one upright witness sights the crescent of Ramadan4, whereas for all other months, the testimony of no fewer than two upright witnesses is accepted.
244—The intention for an obligatory fast must be made during the preceding night.
245—As for a voluntary fast, it may be observed with an intention made during the day.
246—A sick person who would be harmed by fasting and a traveler may either refrain from fasting or observe it5.
247—Menstruating women and women experiencing postpartum bleeding are forbidden to fast and must make up the missed fasts.