Rule 901.8

Planechase games have an inherent triggered ability known as the “planeswalking ability.” The full text of this ability is “Whenever you roll {PW}, planeswalk.” (See rule 701.24, “Planeswalk.”) This ability has no source and is controlled by the player whose planar die roll caused it to trigger. This is an exception to rule 113.8.

Find this number in the current Rules