Journey into Mystery #108 Sep 1964
Doctor Strange and the Avengers (Iron Man, Giant-Man and the Wasp) crossover in
"At the Mercy of Loki, Prince of Evil!"
Thor is called to the aid of Doctor Strange,

and as Dr. Blake saves his life after a crippling mystic attack;

Loki takes advantage of a misunderstanding between Thor and Odin to come to Earth and kidnap Jane Foster;

Thor finds Loki and defeats him, while a grateful Doctor Strange saves Jane.

Script by Stan Lee, pencils by Jack Kirby, inks by Chic Stone