Maliks Muwatta Book 18, Hadith Number 3.

Section : Sighting the New Moon for Beginning and Ending the Fast of Ramadan.

Yahya related to me from Malik from Thawr ibn Zayd ad-Dili from Abdullah ibn Abbas that the Messenger of Allah, may Allah bless him and grant him peace, once mentioned Ramadan and said, “Do not start the fast or break it until you see the new moon. If the new moon is obscured from you, then complete a full thirty days.”