Sahih Al Bukhari Hadith Number 1900

Narrated Ibn `Umar: I heard Allah’s Messenger (peace be upon him) 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.”

30  Fasting
Hadith number 1900
Volume no 3
Chapter: 5 (Should it be said “Ramadan” or “the month of Ramadan”? And whoever thinks that both are permissible)