— Babe Ruth American baseball player 1895 - 1948
Disputed, The earliest quotes similar to this are presented as unattributed folk wisdom, such as this example from 1959:
As Brother Allen of Newsweek indicated, it has been fun, but don't try to rest on your laurels. Always remember, “YESTERDAY’S HOME-RUN DOESN’T COUNT IN TODAY’S GAME,” and today’s game is well under way.
Source: F. N. Abbott, "On Your Marks", in [The Palm, vol lxxix, no. 1 (February 1959), Harry L., Bird (ed.), 1959, Champaign, IL, Alpha Tau Omega, 17, http://hdl.handle.net/2027/uiuc.1744313v0079?urlappend=%3Bseq=19]
The quote does not begin to be attributed to Babe Ruth until the 1980s, nearly 30 years after its first appearance.