On the last day of the feast, the great day, Jesus stood up and proclaimed, “If any one thirst, let him come to me and drink. He who believes in me, as the scripture has said, ‘Out of his heart shall flow rivers of living water.'” Now this he said about the Spirit, which those who believed in him were to receive; for as yet the Spirit had not been given, because Jesus was not yet glorified.
St Basil the Great (d. 379AD) comments:
‘He who believes in me,’ as the Scriptures says, “From within him there shall flow rivers.” ’ And again, ‘If anyone drinks of the water which I give, it will become in him a fountain of water, springing up unto life everlasting.’ This river, accordingly, makes all the city of God at once joyful, that is to say surely, the Church of those who hold to a heavenly manner of life. Or, every creature endowed with intelligence, from celestial powers even to human souls, must be understood as the city made joyful by the inflowing of the Holy Spirit.” (The Fathers of the Church: Exegetic Homilies, p 302)