Historical Rules · 1 February 2010
Rule 510.1f
Official source text from the 1 February 2010 Rules release
Once a player has assigned combat damage from each attacking or blocking creature he or she controls, the total damage assignment is checked to see if it complies with the above rules. If it doesn't, the combat damage assignment is illegal; the game returns to the moment before that player began to assign combat damage. (See rule 714, "Handling Illegal Actions").