AO - Is there sin unto death?

AO - Is there sin unto death? [∞]

▸ ± 1 John 5:16 "If any man see his brother sin a sin which is not unto death, he shall ask, and he shall give him life for them that sin not unto death. There is a sin unto death: I do not say that he shall pray for it.

Yes. It says so. "There is a sin unto death." The real question is:

∞ Q: What is the OT source for saying that there is a sin unto death? Answer: