POSSESSION

Player 1 and Player 2 go up for the rebound; Player 1 grabs the ball with one hand, but, at the same time Player 2 slaps Player 1's hand with the ball forcing the ball to go out of bound. Whose ball is it. Player 1 was the last person who touch the ball and all Player 2 did was hit Player 1's hand forcing the ball to go out of bound.

I'm not sure of the exact "technical ruling". But in almost all cases, the ref will award the ball to player 1's team. It's almost impossible for the ref to tell if player 2 hit the ball, hand, or what.

A ref could use the rule that the ball is considered part of the hand and the hand can accidentally be slapped with no foul. Since Player 2 slapped Player 1's hand while it was on the ball, I'd view that the same as Player 2 slapping the ball OOB and award possession to Player 1's team.