Rule 806.6a

A player who is dissatisfied with his or her initial hand may mulligan. First, each player on the starting team declares whether or not he or she will take a mulligan. Then the players on each other team in turn order do the same. Teammates may consult during this process, but a player can't see the result of his or her teammate's mulligan before deciding whether to take a mulligan at the same time. A player may take a mulligan even after his or her teammate has decided to keep his or her opening hand. Once each player has made a declaration, all players who decided to take mulligans do so at the same time. To take a mulligan, a player shuffles his or her hand back into his or her library, then draws a new hand. In a multiplayer game, the first time a player takes a mulligan, his or her new hand is seven cards; each subsequent mulligan that player takes results in a hand of one fewer cards than the last. If a player kept his or her hand of cards, those cards become the player's opening hand, and that player may not take any further mulligans. This process is then repeated until no player takes a mulligan. (Note that if a player's hand size reaches zero cards, that player must keep that hand.)

Open rule in this release