Say you have hit your ball into a good position a couple of yards away from the hole. Your playing partner takes their shot and their ball knocks yours into the hole.
What would happen in this scenario?
If I understand well this is the case of Rule 18-5.
According to this rule the player must replace his ball back to its original position so on the green.
(See related FAQ under Rule 19) The other player must play his ball as it lies. There is no penalty to either player in either match play or stroke play. In stroke play only, if the player's ball lay on the putting green prior to the stroke, he would incur a penalty of two strokes.
According to the PGA Rulebook
Ball at rest struck by another ball
If a ball in play and at rest is moved by another ball in motion after a stroke, the moved ball must be replaced.