It was in the end of season 6, after Leonard leaves for his sea expedition with Stephen Hawking.

AFAIR, it was after they return from the airport(?) when Penny and Raj suddenly realize that Raj is able to talk with her despite not having drunk any alcohol for three days.

Edit: Sorry, I saw [the comment above](http://movies.stackexchange.com/q/30668/20690#comment45200_30668) too late, but to be complete here, it was [The Bon Voyage Reaction.](http://bigbangtheory.wikia.com/wiki/The_Bon_Voyage_Reaction)