Maliks Muwatta Book 018, Hadith Number 005.

Section : Making the Intention to Fast Before Dawn.

Yahya related to me from Malik from Nafi that Abdullah ibn Umar used to say, "Only some one who makes the intention to fast before dawn (actually) fasts."

Yahya related to me from Malik from Ibn Shihab that A'isha and Hafsa, the wives of the Prophet, may Allah bless him and grant him peace, also said that.