Historical Rules · 17 June 2011
Rule 103.4d
Official source text from the 17 June 2011 Rules release
The Commander casual variant uses an alternate mulligan rule. Each time a player takes a mulligan, rather than shuffling his or her entire hand of cards into his or her library, that player exiles any number of cards from his or her hand. Then the player draws a number of cards equal to one less than the number of cards he or she exiled this way. Once a player keeps an opening hand, that player shuffles all cards he or she exiled this way into his or her library.