Then I saw an angel coming down from heaven, holding in his hand the key to the abyss and a huge chain.
References for Revelation 20:1
He seized the dragon, the old snake, who is the devil and Satan, and bound him for a thousand years.
He threw him into the abyss, then locked and sealed it over him. This was to keep him from continuing to deceive the nations until the thousand years were over. After this he must be released for a little while.