Chapter 91.
198 Petilianus said: "But you scatter thorns and tares, not seeds of grain so that you ought to be burned together with them at the last judgment. We do not utter curses; but every thorny conscience is bound under this penalty by the sentence which God has pronounced."
199. Augustine answered: Surely, when you mention tares, it might bring to your minds the thought of wheat as well; for both have been commanded to grow together in the field until the harvest. But you fix the eye of malice fiercely on the tares, and maintain, in opposition to the express declaration of Christ, that they alone have grown throughout the earth, with the exception of Africa alone.
Source: Answer to Letters of Petilian, Bishop of Cirta (New Advent)