Best I could find, although behind the WSJ paywall: https://www.wsj.com/articles/SB10001424052970204319004577084....

(Archive link: https://archive.is/fKzDr)

There aren't any formal rules that he lives and dies by, but read "Buffett: The Making of an American Capitalist" by Lowenstein. Buffett -- or at least Lowenstein captures -- expresses concern for people buying before he does.