John 7:37-39
On the last and greatest day of the festival, Jesus stood and said in a loud voice, “Let anyone who is thirsty come to me and drink. Whoever believes in me, as Scripture has said, rivers of living water will flow from within them.” By this he meant the Spirit, whom those who believed in him were later to receive. Up to that time the Spirit had not been given, since Jesus had not yet been glorified.
This was an idomatic manner of expressing things commonly use din the Middle east of the time, and porbably to this day. Strictly speaking, the living water being the Holy Spirit in the Christian sense is a Johannine innovation. It does occur in the Old Testament, such as in Jeremiah, where God calls himself a living water (i.e. the source of life).
But then you have Ezekiel 14:8 who writes "And in that day living waters will flow out of Jerusalem, half of them toward the eastern sea and the other half toward the western sea...And the LORD will be king over all the earth; in that day the LORD will be the only one, and His name the only one." Clearly this does not correpsond to John's construct very well.
On the other hand we have Isaiah 44:3 where it says "'For I will pour out water on the thirsty land and streams on the dry ground; I will pour out My Spirit on your offspring and My blessing on your descendants."
But he is talking of Israel, of course, so this dioesn't apply either.
And then again Isaiah 55:1 "Ho! Every one who thirsts, come come, buy wine and milk without money and without cost."
Of course this also has nothing to do with Jesus, but was used by the Gospel scribes for their agenda.
Or Isai 58:11 "And the LORD will continually guide you, and satisfy your desire in scorched places, and give strength to your bones; and you will be like a watered garden, and like a spring of water whose waters do not fail."
And Joel 2:28 says "It will come about after this that I will pour out My Spirit on all mankind; and your sons and daughters will prophesy, your old men will dream dreams, your young men will see visions."
Pouring the spirit on all mankind obviously is something that does not describe the pouring of the Holy Spirit on the believers.
But one can see how cherry-picked OT phrases were used to advance Christian agenda.