Augustine of Hippo

[Psalm 4:7] "You have put gladness into my heart." Gladness then is not to be sought without by them, who, being still heavy in heart, "love vanity, and seek a lie;" but within, where the light of God's countenance is stamped. For Christ dwells in the inner man, [Ephesians 3:16-17] as the Apostle says; for to Him does it appertain to see truth, since He has said, "I am the truth." [John 14:6] And again, when He spoke in the Apostle, saying, "Would you receive a proof of Christ, who speaks in me?" [2 Corinthians 13:3] He spoke not of course from without to him, but in his very heart, that is, in that chamber where we are to pray.