Musannaf Ibn Abi Shayba 21:761

Abu Usamah narrated to us, he said: Abdullah ibn al-Walid narrated to me, from Ziyad, from al-Hasan, he said: Whoever says, when the mu'adhdhin says, "I bear witness that there is no god but Allah, and I bear witness that Muhammad is the Messenger of Allah," and "I bear witness along with whoever bears witness," he shall have the reward of whoever bears witness and whoever does not bear witness.