{ If you are in doubt about what We have sent down to you, just ask those who have been reading the book before you } Yunus 94

From Mujahid, regarding His word { Just ask those who have been reading the book before you } ; he said: These are the People of the Book.
Tafsir Al-Tabari

The word of the Most High { Just ask those who have been reading the book before you } : These are the Jews and the Christians.