Musnad Ahmad 31:859

'Uthman ibn 'Umar narrated to us, Malik and Yunus ibn Yazid informed us, from al-Zuhri, from 'Ata' ibn Yazid al-Laythi, from Abu Sa'id al-Khudri, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "When you hear the mu'adhdhin" — and Malik said: "the caller" — "then say the like of what he says." Malik added: "the mu'adhdhin."