Musannaf Ibn Abi Shayba 22:244

Waki’ narrated to us, from Sufyan, from Salamah ibn Kuhayl, from Hujr ibn ‘Isa, from Wa’il ibn Hujr, who said: I heard the Prophet, may Allah bless him and grant him peace, recite "wa la ad-dallin," and he said, "Amin," extending his voice with it.