“Be Prepared in Mind by having disciplined yourself to be obedient to every order, and also by having thought out beforehand any accident or situation that might occur, so that you know the right thing to do at the right moment, and are willing to do it.

Be Prepared in Body by making yourself strong and active and able to do the right thing at the right moment, and do it.”

As quoted by John M. Kost http://www.mackinac.org/bio.aspx?ID=104 (25 July 1995) in S. 946, the Information Technology Management Reform Act of 1995: hearing before the Subcommittee on Oversight of Government Management and the District of Columbia of the Committee on Governmental Affairs (1996).
This appears to derive from a 1910 advertisement by writer Alfred Henry Lewis for a forthcoming series of biographical articles about Roosevelt: "All activity, Mr. Roosevelt has often shown that it is better to do the wrong thing than do nothing at all. In politics this last is peculiarly true. The best thing is to do the right thing; the next best is to do the wrong thing; the worst thing of all things is to stand perfectly still". (e.g. in La Follette's Magazine https://books.google.com/books?id=RV4CAAAAIAAJ&pg=PA183&dq=%22best+thing%22+%22right+thing%22+%22worst+thing%22+nothing&hl=en&sa=X&ved=0ahUKEwjNksu-nZrMAhVDy2MKHSl1Df8Q6AEIIzAB#v=onepage&q=%22the%20best%20thing%20is%20to%20do%20the%20right%20thing%22&f=false (28 May 1910)
Disputed

Source: The Last Song

A Short History of England (1917)

We The Living (1936)
Source: We The Living Part One Chapter 6

Quoting her mother's statement after her son's birth, in I Know Why the Caged Bird Sings (1969)

As quoted in The Forbes Book of Business Quotations (2007) edited by Ted Goodman, p. 175