The flashback episode "Pre-Witched" after Penny has just died from a heart attack. A scene after the funeral has all sisters in the living room together. Piper tries to keep things optimistic and offers to make everyone dinner but Prue rebuffs her to meet her fiancee while Phoebe informs the two of them that she's moving out, saying there's nothing there for her anymore. Both Prue and Phoebe walk out of the room leaving Piper alone trying to hold back her tears. Contrasted with a Heartwarming Moment at the end of the episode in the present when Piper decides not to move out of the house.
Piper: "But it feels good that forever isn't today".