From 26 CFR: see link: https://www.law.cornell.edu/cfr/text/26/1.408A-4

A-7. (a) Any amount that is converted to a Roth IRA is includible in gross income as a distribution according to the rules of section 408(d)(1) and (2) for the taxable year in which the amount is distributed or transferred from the traditional IRA. Thus, any portion of the distribution or transfer that is treated as a return of basis under section 408(d)(1) and (2) is not includible in gross income as a result of the conversion.