Musnad Abu Ya'la al-Mawsili 219:20

Muhammad ibn Bashar narrated to us, Muhammad narrated to us, Shu'bah narrated to us, from Abu Bishr, from Abu al-Malih ibn Usamah, from Umm Habibah, from the Prophet, peace and blessings of Allah be upon him: That when he would hear the mu'adhdhin, he would say as the mu'adhdhin says, until he would fall silent.