Musannaf Ibn Abi Shayba 16:183

Abu Bakr narrated to us, he said: Hushaym narrated to us, he said: Mughira narrated to us, from Ibrahim, who said: Umar wrote: "Mention the name of Allah over the cheese and eat." Ibrahim said: When we traveled to these mountains and saw what we saw of the practices of the non-Arabs (Ajam), we disliked it, except for asking about it.