Yahya related to me from Malik from Abdullah ibn Dinar fromAbdullah ibn Umar that the Messenger of Allah, may Allah bless him andgrant him peace, said, "A month has twenty-nine days in it. Do notstart the fast or break it until you see the new moon. If the new moonis obscured from you, then work out (when it should be)."
USC-MSA web (English) reference: Book 18, Hadith 2
Arabic reference: Book 18, Hadith 634