The Free Prebyterian online copy of the Westminster Confession quotes three verses in support of the statement :
neither is God the author of sin,b
Free Presbyterian Church Documents
The first two are from James :
Let no man say when he is tempted, I am tempted of God: for God cannot be tempted with evil, neither tempteth he any man:
James 1:13
Every good gift and every perfect gift is from above, and cometh down from the Father of lights, with whom is no variableness, neither shadow of turning.
James 1:17
The third is from John :
This then is the message which we have heard of him, and declare unto you, that God is light, and in him is no darkness at all.
I John 1:5
I am not certain if these scriptures are added by the Free Presbyterian Church to their online presentation or if they are original to the Westminster Confession.
(If someone can comment and link on this, I shall edit accordingly.)