Maliks Muwatta Book 3, Hadith Number 8.
Section : The Call to Prayer.
Yahya related to me from Malik that he had heard that the muadhdhin came to Umar ibn al-Khattab to call him to the subh prayer and found him sleeping, so he said, “Prayer is better than sleep,” and Umar ordered him to put that in the adhan for subh.