"The prophets, therefore, did not openly behold the actual face of God, but [they saw] the dispensations and the mysteries through which man should afterwards see God" (Irenaeus, Against Heresies IV.20.10).
Then Irenaeus writes: "If, then, neither Moses, nor Elias, nor Ezekiel, who had all many celestial visions, did see God; but if what they did see were similitudes of the splendour of the Lord, and prophecies of things to come; it is manifest that the Father is indeed invisible, of whom also the Lord said, 'No man hath seen God at any time' " (Ibid. IV.20.11).
He's discussing Exodus 33:19-22. Interesting how he applies this text to the Father. Furthermore, when he quotes the Lord's words, evidently referring to John 1:18, he likewise applies these words to the Father.