As I walked out the door toward my freedom, I knew that if I did not leave all the anger, hatred, and bitterness behind, that I would still be in prison.
Nelson Mandela (1918-2013) South African revolutionary, politician, statesman
(Attributed)
On his release from 27 years behind bars. Quoted by Hillary Clinton from a conversation she had with him.
Hillary Rodham Clinton quotes Mandela as saying this to her on 11 Feb 1990 (the day Mandela was released from prison) in her book “Living History” (2003) [http://bit.ly/2pDJknS].
As I read that, he told her the quote regarding his release on that date. The quote itself, in context, appears to have come some time after (“when I got to know Mandela better”) the introductory scene (his inaugural dinner). Regardless, you’ve found as close to a primary source for the quotation as I can see.
Having gone back and re-read, I completely agree with you.