“Successful people do two things that many other people don’t: They initiate action, and they finish what they start.”

Alleged source in Plato unknown. Earliest occurrence to have been located is a Tweet from 2011 https://twitter.com/ochocinco/status/93332058864238592.
Disputed

Interview with David Shankbone (2007)

Source: The Winter People

The Piano Teacher (1988)

Positive Thinking Every Day : An Inspiration for Each Day of the Year (1993), "April 13"
Earlier variant: People become really quite remarkable when they start thinking that they can do things. And those who have learned to have a realistic, nonegotistical belief in themselves, who possess a deep and sound self-confidence, are assets to mankind, too, for they transmit their dynamic quality to those lacking it.
‪You Can If You Think You Can‬ (1987), p. 84

Revolution 2020 (2011)

Interview by Lindsay Baker, "The Unsinkable Ian Brown" http://www.guardian.co.uk/music/2002/feb/02/shopping.popandrock?INTCMP=SRCH, The Guardian, 2 February 2002, retrieved 2011-08-13

From interview with Anshul Chaturvedi