Rule 118.13 · Costs
Some costs contain mana symbols that can be paid in multiple ways. These include hybrid mana symbols and Phyrexian mana symbols.
Subrules
If the mana cost of a spell or the activation cost of an activated ability contains a mana symbol that can be paid in multiple ways, the choice of how to pay for that symbol is made as its controller proposes that spell or ability (see rule 601.2b).
If a cost paid during the resolution of a spell or ability contains a mana symbol that can be paid in multiple ways, the player paying that cost chooses how to pay for that symbol immediately before they pay that cost.
If the cost associated with a special action contains a mana symbol that can be paid in multiple ways, the player taking the special action chooses how to pay for that symbol immediately before they pay that cost.