Musnad Ahmad 5:401

Abdullah narrated to us, Muhammad ibn al-Minhal, the brother of Hajjaj ibn Minhal, narrated to me, Abd al-Wahid ibn Ziyad narrated to us, from Abd al-Rahman ibn Ishaq, Abu Sa'id narrated to me, from Abd al-Rahman ibn Abi Layla, who said: When Ali ibn Abi Talib would hear the mu'adhdhin calling the adhan, he would say as he said. When he said: "I bear witness that there is no god but Allah, and I bear witness that Muhammad is the Messenger of Allah," Ali would say: "I bear witness that there is no god but Allah, and I bear witness that Muhammad is the Messenger of Allah, and that those who denied Muhammad are the liars."