The prayers of the twelve and of the multitude were apparently in spiritual preparation for the transformational event witnessed at the end of chapter 19. With the conclusion of that event, the intense purpose of this mass prayer is over, and the Savior instructs them to cease this active prayer. Of course they are not to cease the yearning of their heart, so we have the admonition to continue to pray in their hearts.
Textual: The 1830 edition did not have a chapter break at this point. Without the break, the command to cease praying has a better context of the continual prayers of the twelve and the multitude.