Rule 508.1c

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.

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