Incomplete illegal forward pass

[mikesears]

1/10 from A-20. A7 drops back to throw a pass and is under heavy rush. He steps forward and begins to run and crosses the neutral zone. When he is at the A-23, he throws and incomplete forward pass towards A88.

If for some stupid reason B wanted to decline the penalty, where would the succeeding spot be?

I'm at work and without my rulebook so I can't check this for myself.

Thanks!