It's found here: https://www.irs.gov/taxtopics/tc502

It reads: If you're self-employed and have a net profit for the year, you may be eligible for the self-employed health insurance deduction. This is an adjustment to income, rather than an itemized deduction, for premiums you paid on a health insurance policy covering medical care, including a qualified long-term care insurance policy for yourself, your spouse, and dependents. The policy can also cover your child who is under the age of 27 at the end of 2018 even if the child wasn't your dependent. See Chapter 6 of Publication 535, Business Expenses for eligibility information. If you don't claim 100% of your paid premiums, you can include the remainder with your other medical expenses as an itemized deduction on Form 1040, Schedule A.pdf.

thank you

The citation you are looking for is §162(l).