Maliks Muwatta Book 18, Hadith Number 2.

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

Yahya related to me from Malik from Abdullah ibn Dinar from Abdullah ibn Umar that the Messenger of Allah, may Allah bless him and grant him peace, said, “A month has twenty-nine days in it. Do not start the fast or break it until you see the new moon. If the new moon is obscured from you, then work out (when it should be).”