Yahya related to me from Malik from Thawr ibn Zayd ad-Dili fromAbdullah ibn Abbas that the Messenger of Allah, may Allah bless himand grant him peace, once mentioned Ramadan and said, "Do not startthe fast or break it until you see the new moon. If the new moon isobscured from you, then complete a full thirty days."
USC-MSA web (English) reference: Book 18, Hadith 3
Arabic reference: Book 18, Hadith 635