"can someone point me to the regulation for this?"

google it; it's everywhere:

how to correct traditional ira excess contribution

Pub 590-A: https://www.irs.gov/publications/p590a

But I like these types of listings: https://www.investopedia.com/articles/retirement/04/042804.asp

You can leave it and keep paying that 6% tax; you can remove it and pay the income tax on the excess + earnings; you can pay that 6% tax until you manage to fully allocate it across upcoming years ("use it up") which can be risky in a moving earnings timeframe (they might not qualify to use it, next year).