Musannaf Ibn Abi Shayba 2:244

Abu Bakr narrated to us, he said: Ibn ‘Uyaynah narrated to us, from ‘Amr, from Abu Ja’far Muhammad ibn ‘Ali, that the Prophet (may Allah bless him and grant him peace), when he would hear the voice of the caller, he would say: "I bear witness that there is no god but Allah," and he would say: "And I." And when he (the caller) said: "I bear witness that Muhammad is the Messenger of Allah," he would say: "And I."