Musannaf Ibn Abi Shayba 35:346

Waki’ narrated to us, from Sufyan, from Salamah ibn Kuhayl, from Hujr ibn ‘Anbas, from Wa’il ibn Hujr, who said: I heard the Prophet, may Allah bless him and grant him peace, recite "wa la al-dallin," and he said, "Amin," extending his voice with it. And he mentioned that Abu Hanifah said: The Imam does not raise his voice with "Amin," and those behind him say it.