Abdullâh b. ‘Abbâs – Allâh be pleased with them – said:

O Muslims, how can you ask the People of the Book [about their scripture] when the Book of Allâh that was revealed to your Prophet – Allâh’s peace and blessings be upon him – is the most recent news of Allâh; you recite it complete and unaltered? Allâh has informed you that they replaced what Allâh wrote and changed the Book with their hands and said ‘This is from Allâh,’ to purchase with it a measly price (Quran 2:79). Does not the knowledge that has come to you forbid you from asking them? By Allâh, we have not seen a single man among them asking you about what was revealed to you.

Al-Bukhârî, Al-Sahîh hadith 2685, Chapter on not asking the Polytheists for testimonies or anything else. He also records it in other chapters.