Rule 903.8

The Commander 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 face down. Then the player draws a number of cards equal to one less than the number of cards he or she exiled this way. That player may look at all cards exiled this way while taking mulligans. Once a player keeps an opening hand, that player shuffles all cards he or she exiled this way into his or her library.

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