Is there any Hadith of the Prophet (peace be upon him) that what we should say when we pass by a non-Muslim grave?
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Imam Ibn Maajah narrated that the Prophet said: "Whenever you pass by a grave of a non-Muslim, give him tidings of Hellfire.” (The chain of this Hadeeth is sound as mentioned in Majma’ Az-Zawaa-id).
Allah knows best.
You can search for fatwa through many choices