Maliks Muwatta Book 3, Hadith Number 15.
Section : The Duration of the Meal before Dawn (Sahur) in Relation to the Adhan.
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, “Bilal calls the adhan whilst it is still night so eat and drink until Ibn Umm Maktum calls the adhan.”