Yankees retire Mariano Rivera's number
Paul O'Neill stunned Yankees are retiring his number
The Yankees have a lot of retired numbers. Other teams should try to be more like them. - NBC Sports
As Paul O'Neill's No. 21 Is Retired, One Last Kick of a Water Cooler - The New York Times
Yankees retire Paul O'Neill's No. 21 jersey, Cashman booed
Yankees retired numbers: A look at all of the jersey numbers players cannot wear in the Bronx
Where Derek Jeter ranks among Yankees' retired single-digit numbers
Retired Uniform Numbers in the National League
New York Yankees - Wikipedia
Bill Russell's No. 6 Joins Rare Class of Retired Jersey Numbers - The New York Times