That's in pub 527 at https://www.irs.gov/pub/irs-pdf/p527.pdf
Page 9, Table 2-1. Note the table shows 27.5 years under GDS and 30 years under ADS. Note the superscript "1" for the 30 years. Referring to note 1 below the table states 40 years for property placed in service before January 1, 2018.
