*Reprinting AVENGERS (1963) #4. *Source: marvel.com
"Captain America Joins... the Avengers!"
The Avengers, while hunting for Namor in Arctic waters, find a body frozen in a block of ice. They recover it, and let the ice melt, discovering that the figure is clad in the iconic costume of the World War II freedom fighter, Captain America, long considered by the world to be dead. Miraculously, once the ice is melted, the man revives, although disorientated. He is, in fact, Steve Rogers, the original Captain America, who had slept unaged in suspended animation all these years. The Avengers invite him to join them, and Cap agrees.
Reprinted in Avengers Annual #3.