The section of the Apple HIG about specific button labels is "UI Element Guidelines: Windows > Alerts": https://developer.apple.com/library/mac/#documentation/usere...

Microsoft's interface guidelines also give similar advice: http://msdn.microsoft.com/en-us/library/aa511268.aspx#respon...

bonus: Here is a bug report whose comments includes a long explanation that I wrote, on why we should label the buttons "Save/Don't Save/Cancel" whether the platform standard says to or not: http://sourceforge.net/apps/ideatorrent/notepad-plus/ideator...