ANSWER:
I don’t know of such a hadeeth. If an innovator or a mushirk repented before dying, Allah will forgive them.
There is a hadeeth where the Prophet salla Allahu alaihi wa Sallam had said: Allah had blocked repentance from every innovator.
This doesn’t mean that Allah doesn’t accept his repentance. It means that because he thinks he is doing well, he wouldn’t repent!