Ambrose (c. 339-97) from a sermon to his congregation:
"Trust to no one, to guide you, but where the light of that lamp [i.e. Scripture] goes before. For where you think it shines, there is a whirlpool; it seems to shine, but it defiles; and where you think it is firm or dry, there it is slippery. And, moreover, if you have a lamp, the way is long. Therefore let faith be the guide of your journey; let the divine Scripture be your path. Excellent is the guidance of the heavenly word. From this lamp light your lamp; that the eye of your mind, which is the lamp of your body, may give light."
Source: Translation by William Goode, The Divine Rule of Faith and Practice, 2nd ed., 3 Vols. (London: John Henry Jackson, 1853), Vol. III, p. 148.
Latin text: nulli credas tuum, nisi praeeunte lucernae istius luce, processum. Nam ubi putas quod luceat, gurges est; videtur lucere sed polluit; et ubi putas solidum esse vel siccum, ibi lubricum est. Sed et si lucerna tibi, iter longius sit. Sit ergo fides tibi itineris tui praevia, sit tibi iter Scriptura divina. Bonus est coelestis ductus eloquii. Ex hac lucerna accende et tu lucernam; ut luceat interior oculus tuus, qui lucerna est tui corporis. [In Psalmum David CXVIII, Expositio, Sermo 14, §11, PL 15:1394].
For an excellent compilation of quotes of the Church fathers teaching on the primacy, sufficiency and ultimate authority of Scripture, get a copy of Holy Scripture:The Ground and Pillar of Our Faith Vol III- The Writings of the Church Fathers Affirming the Reformation Principle of Sola Scriptura.