Finding tax law?

I have some gifted property that I have taken care of for 12 years and recently started renting.  When I inquired about coming up with a cost basis for the property I was told... For gifted property, your cost basis is the "cost basis of the donor on the date of the gift" PLUS your improvements and special assessments minus any casualty losses you took as a tax deduction. 

My question is, "Where do I find that law"?

Thank you so much for your help.