It sounds like your husband had some delirium from the anesthesia. When people undergo a complicated surgery, sometimes they get confused, agitated, or delirious afterwards. While, it is interesting that
he thinks he went to Russia, it seems like he absolutely believes that this really happened. This is a delusional thought. A hallucination would be to hear and see things that are not there.
What I would try to do first is reassure him that while he thinks he went to Russia, he really was in the hospital recovering from a procedure. I would not laugh or joke about it, as the experience was very real to him. Tell him he was reacting to the medications they gave him, but not to worry, as he recovered well. Hopefully, this will make him understand.
One last thought: If he continues to have any new delusions, I would definitely let his health care provider know, as this can be a sign of something more serious.