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)
The only citation of Freyd in that piece (P. Freyd, Relative homological algebra made absolute, Proc. Nat. Acad. Sci. U.S.A. 49 (1963), 19-20) does not appear to be the source of this quotation (if you can call it that) so the true origin may have to remain unknown.