Maliks Muwatta Book 3, Hadith Number 2.
Section : The Call to Prayer.
Yahya related to me from Malik from Ibn Shihab from Ata ibn Yazid al-Laythi from Abu Said al-Khudri that the Messenger of Allah, may Allah bless him and grant him peace, said, “When you hear the adhan, repeat what the muadhdhin says.”