20880302_ferences__July_2014_FIDE_Laws___FIDE_rule
[0] 10. [USCF rule] When castling, the player may touch either the king or the rook first. [FIDE rule] 
When castling, the player must touch the king first (or the king and rook at the same time). If the player touches the rook first, castling with that rook is not allowed, and the touch move rule is applied to the rook.
The United States Chess Federation - USCF - FIDE Rule Differences (July 2014 FIDE Laws)
FIDE rule