"On the last day of the feast, the great day, Jesus stood up and cried out, 'If anyone thirsts, let him come to me and drink. Whoever believes in me, as the Scripture has said, "Out of his heart will flow rivers of living water." 'Now this he said about the Spirit, whom those who believed in him were to receive, for as yet the Spirit had not been given, because Jesus was not yet glorified." (ESV)
A promise foretold in the scriptures
"The reference is not to any single passage, but to the spirit of the Scripture, notably such passages as Isaiah 55:1; 58:11; Psalms 36:8-9." -B. W. Johnson "...referring not to any particular passage, but to such as Isaiah 58:11; Joel 3:18; Zechariah 14:8; Ezekiel 47:1-12; in most of which the idea is that of waters issuing from beneath the temple, to which our Lord compares Himself and those who believe in Him." -Jamieson, Fausset, and Brown
The Spirit had not yet been given
"The first and second chapters of the Book of Acts is the best comment upon this passage. When Jesus ascended to the right hand of the Father and was glorified, he sent forth the Spirit upon his apostles on the day of Pentecost, and the apostles in turn promised the gift of the Spirit to all who would believe, repent, and be baptized." -J. W. McGarvey (Fourfold Gospel) "John's statement is about the redemptive-historical ministry of the Spirit, not His existence."