Season 10: Episode 226: The One Where Chandler Gets Caught
Rachel and Phoebe spot Chandler with a blonde stranger; Phoebe thinks he's having an affair. They follow the pair to a house outside the city. They tell the rest of the gang, and then confront Monica and Chandler, forcing the revelation that Monica and Chandler are trying to move to a house outside the city to raise their child. In attempts to convice Chandler and Monica to stay, their friends remind them of all the great memories they've had in their apartment in the city (shown as clips from previous episodes).