Historical Rules · 25 January 2019
Rule 805.10b
Official source text from the 25 January 2019 Rules release
As the declare attackers step begins, the active team declares attackers. For each attacking creature, the attacking team announces which defending player or planeswalker that creature is attacking. The active team has one combined attack, and that set of attacking creatures must be legal as a whole. See rule 508.1.