Musnad Abu Ya'la al-Mawsili 42:216

Zuhayr narrated to us, Abd al-Rahman narrated to us, Malik narrated to us, from al-Zuhri, from Ata’ ibn Yazid al-Laythi, from Abu Sa’id al-Khudri, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "When you hear the call, say the like of what the mu'adhdhin says."