Chapter 30: Fasting
Narrated Ibn 'Umar: I heard Allah' s Messenger (ﷺ) saying, "When you see the crescent (of the month of Ramadan), start fasting,
and when you see the crescent (of the month of Shawwal), stop fasting; and if the sky is overcast (and
you can' t see it) then regard the month of Ramadan as of 30 days."
Sahih al-Bukhari 1900