SUPERNATURAL Season 6 Episode 11 : Appointment in Samarra
Dean seeks out Death to help get Sam's soul back. Death tells Dean he will help Sam if Dean will agree to act as Death for 24 hours. Meanwhile, Sam decides he doesn't want his soul back and asks Balthazar for a protective spell to keep his soul out of his body. Balthazar tells Sam he needs the blood of his father for the spell to work and since John Winchester is dead, Sam decides Bobby is the next best thing.