Acts 16:25-26:

And at midnight Paul and Silas prayed, and sang praises unto God: and the prisoners heard them.
And suddenly there was a great earthquake, so that the foundations of the prison were shaken: and immediately all the doors were opened, and every one's bands were loosed.

See also:

Does the Prayer of Agreement Prove That God Responds More Readily If More People Pray About the Same Thing?
Notes on Matthew 27:52-53
Notes on Acts 4:29-33