Avatar: The Last Airbender S00 E07 Escape From the Spirit World

"Water. Earth. Fire. Air."

After being shot by Azula's lethal lightning attack, Aang falls into a deep slumber. Escape from the Spirit World explains that while Aang's mind and body were immobile, his soul was trapped in the Spirit World. He encounters Princess Yue, the Moon Spirit, who informs him that if he does not reconnect with his past four lives, the Avatar Cycle will end.