Here is a somewhat representative sample of the ways in which Augustine expresses this concept. Take your pick.
De gratia Christi et de peccato originali 1.24 (CSEL 42: 144). quomodo est ergo gratia, si non gratis datur? [How then is it grace, if it is not given gratuitously?]
De gratia Christi et de peccato originali 1.34 (CSEL 42: 152). isti autem libero arbitrio sic applicant fidem, ut fidei uidelicet reddi uideatur non gratuita, sed debita gratia, ac per hoc iam nec gratia, quia nisi gratuita non est gratia. [But these Pelagians attribute faith to free will in such a way that it seems that grace is rendered to faith not gratuitously, but as something owed, and for this reason it is no longer grace, because it is not grace unless it is gratuitous.]
Contra Iulianem 3.2 (PL 44: 702). nouerant uos dicere, gratiam dei secundum merita nostra dari: ut iam gratia non esset gratia, quia non gratis daretur, sed secundum debitum redderetur. [They had known that you say that the grace of God is given according to our merits, so that it is no longer grace, because it is not given gratuitously, but repaid as a debt.]
De Praedestinatione Sanctorum 43 (PL 44: 992). nec ullum eorum dari gratis, cum in eis dei gratia, quae non nisi gratuita est, praedicetur. [...nor are any of them given freely, although in them is proclaimed the grace of God, which is not [grace] unless gratuitous.]
Epistle 194.19 (CSEL 57: 191). ipsa gratia nuncupatur non ob aliud, nisi quia gratis datur, [...this is called grace for no reason other than that it is given gratuitously...]
In Iohannis Euangelium Tractatus 3.8 (CCL 36: 24). unde uocatur gratia? quia gratis datur. [Whence is it called grace? Because it is given gratuitously.]
In Iohannis Euangelium Tractatus 10.6 (CCL 36: 104). columba non est uenalis; gratis datur, quia gratia uocatur. [The dove is not for sale; it is given gratuitously, because it is called a grace.]
Enarrationes in Psalmos 31.2.7 (CCL 38: 230). si gratia est, gratis datur. [If it is grace, it is given gratuitously.]
Sermones 100.4 (RB 104: 83). si autem gratia est, gratis datur. [But if it is grace, it is given gratuitously.]
Sermones 299.6 (PL 38: 1371). gratia uocatur, quia gratis datur. [It is called grace, because it is given gratuitously.]