Musannaf Ibn Abi Shayba 3:5583

Waki’ narrated to us, he said: Sufyan narrated to us, from Salamah ibn Kuhayl, from Hujr ibn ‘Anbas, from Wa’il ibn Hujr, he 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.