Historical Rules · 18 June 2010
Rule 508.1c
Official source text from the 18 June 2010 Rules release
The active player checks each creature he or she controls to see whether it's affected by any restrictions (effects that say a creature can't attack, or that it can't attack unless some condition is met). If any restrictions are being disobeyed, the declaration of attackers is illegal. Example: A player controls two creatures, each with a restriction that states "[This creature] can't attack alone." It's legal to declare both as attackers.