Maliks Muwatta Book 018, Hadith Number 001.

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

Yahya related to me from Malik from Nafi from Abdullah ibn Umar that the Messenger of Allah, may Allah bless him and grant him peace, once mentioned Ramadan and said, "Do not begin the fast until you see the new moon, and do not break the fast (at the end of Ramadan) until you see it. If the new moon is obscured from you, then work out (when it should be)."