Yahya related to me from Malik from Nafi from Abdullah ibn Umarthat the Messenger of Allah, may Allah bless him and grant him peace,once mentioned Ramadan and said, "Do not begin the fast until you seethe new moon, and do not break the fast (at the end of Ramadan) untilyou see it. If the new moon is obscured from you, then work out (whenit should be)."
USC-MSA web (English) reference: Book 18, Hadith 1
Arabic reference: Book 18, Hadith 633