Official rule text
34.4 Abilities, including some found on technology cards, may instruct a player to exhaust a card to resolve those abilities. If a card is already exhausted, it cannot be exhausted again. a Passive abilities on an exhausted card are still in effect while that card is exhausted.
View official source
Source: Living Rules Reference and FAQ - Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 34.4