Isaiah 1:11

God is talking to the rebellious people of Judah and warns them that he is sick of their offerings. He later tells them to repent and he will forgive their sins. I think this verse addresses the idea that we can sin and just ask God to forgive us. If our hearts are not in the right place then we are asking to be forgiven even though we aren't really repentant, and God is not going to buy it.