Historical Rules · 29 September 2017
Rule 702.21h
Official source text from the 29 September 2017 Rules release
If an attacking creature becomes blocked by a creature, each other creature in the same band as the attacking creature becomes blocked by that same blocking creature. Example: A player attacks with a band consisting of a creature with flying and a creature with swampwalk. The defending player, who controls a Swamp, can block the flying creature if able. If he or she does, then the creature with swampwalk will also become blocked by the blocking creature(s).