I don't have my text with me, but Kant's view in Religion within the Bounds of Reason Alone is that God necessarily comes to the same conclusions of reason with regards to morality and is not necessary for our moral reasoning. (The same view is implied in some of Kant's earlier moral works -- note the absence of an appeal to theology in Groundwork of the Metaphysics of Morals , Metaphysics of Morals, and Critique of Practical Reason).
To simplify, the source of morality is reason for Kant, and God has reason and thus comes to the same conclusions. Those who need to think of God to come to the conclusions of reason are in Kant's view using a crutch.
Thus, Kant rejects the voluntarist position that God is above morality.
Despite this, Kant still maintains that God is necessary for morality to maintain what is often called the "proportionality thesis". This is the idea that people will receive happiness in proportion to the degree that they are moral. Kant recognized that this was not true in this world and make God a "postulate of moral reason" (at least in the Critique of Practical Reason) in order to make it so that being moral makes sense in terms of fairness (by becoming eternally fair).
