Official rule text
78.4 STEP 2—ANNOUNCE RETREATS: Each player may announce a retreat, beginning with the defender. a A retreat will not occur immediately; the units retreat during the “Retreat” step. b If the defender announces a retreat, the attacker cannot announce a retreat during that combat round. c A player cannot announce a retreat if there is not at least one eligible system to retreat to.
View official source
Source: Living Rules Reference and FAQ - Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 78.4