*CW since some of the recent posts on MO have required little more than googling.*
<hr>
Prior to the book you mentioned, MacLane attributed this saying to Peter Freyd in: 

MacLane, S. (1965). Categorical algebra. *Bulletin of the American Mathematical Society*, 71(1), 40-106.

**Relevant excerpt:** (p. 48)

![enter image description here][1]

With regard to the original language, Eric Wofsey points out that Freyd's [Abelian Categories][2] (1964) begins with this description:

![enter image description here][3]


  [1]: https://i.sstatic.net/33qhN.jpg
  [2]: http://emis.maths.adelaide.edu.au/journals/TAC/reprints/articles/3/tr3.pdf
  [3]: https://i.sstatic.net/pQAuP.jpg