In C.S. Lewis's Letters to Malcolm: Chiefly on Prayer Letter 14 about different ways of conceptualizing God's presence among His creation, balancing between complete otherness and Pantheism. The quote is after this paragraph:
... All creatures, from the angel to the atom, are other than God; with an otherness to which there is no parallel: incommensurable. The very word "to be" cannot be applied to Him and to them in exactly the same sense. But also, no creature is other than He in the same way in which it is other than all the rest. He is in it as they can never be in one another. In each of them as the ground and root and continual supply of its reality. And also in good rational creatures as light; in bad ones as fire, as at first the smouldering unease, and later the flaming anguish, of an unwelcome and vainly resisted presence.
Therefore of each creature we can say, "This also is Thou: neither is this Thou."
Lewisiana.nl has an extensive and well researched Quotations and Allusions in C.S. Lewis, Letters to Malcolm which contains the following entry on that quote, suspecting that the quote's ultimate origin maybe from St. Augustine:
Charles Williams, He Came Down From Heaven (1938) ch. 2, p. 25; and The Descent of the Dove: A short history of the Holy Spirit in the Church (1939), p. 57. Lewis makes further use of these words in chapters 4 and 17. He was using it at least as early as 1942 in letter to Daphne Harwood (CL II, 512), where Walter Hooper mentions the chorus of Williams’s 1936 play Seed of Adam as one possible source.
While Lewis is undoubtedly referring primarily to Charles Williams, the real origin of this saying may be much older. “On the significance and authorship of this prayer, which Charles Williams may have found in St Augustine, see Victor de Waal, ‘The history of Doctrine’, Life of the Spirit, xviii (1964), 533” – thus Alastair Fowler in a note to C. S. Lewis’s posthumously published lectures, Spenser’s Images of Life (1967), p. 134.
Another good 2007 journal article on the same topic Contemplating C.S. Lewis's Epistemology by Norbert Feinendegen also mentioned St. Augustine as source:
This last formula, "This also is Thou: neither is this Thou", Lewis borrowed from another of his friends, Charles Williams (who may in turn have read it in St. Augustine) (Lewis, "Williams and the Arthuriad" 151). Lewis accepted this concept as a necessary corrective to every purely affirmative theology (which stressed the unity of God and Man), as well as to every purely negative theology (which laid the stress on the otherness of God and Man) (Spenser's Images of Life 134).
Question: Where in St. Augustine's work can we find this quote "This also is Thou: neither is this Thou" ?