CW since some of the recent posts on MO have required little more than googling.
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)
With regard to the original language, Eric Wofsey points out that Freyd's Abelian Categories (1964) begins with this description:
