Cuirassiers ignoring one hit from ranged fire

I saw on some units reference tables, and even on this website, that Cuirassiers will ignore one hit from infantry ranged fire. That make sense. But I was unable to find any mention of that in the rulebook. Can someone tell me the origin - and so certify the legitimacy - of this rule? Thanks.

Yes, it's right. I think you should find it in the core rules at Cuirassiers description.

I finally got it, thanks!