Rivers of Living Water

John 7:38 is talking about speaking in tongues. Proverbs 10:11a clause says The mouth of a righteous man is a well of life

Proverbs 18:4 says The words of a man's are as deep waters, and the wellspring of wisdom as a flowing brook.

Isaiah 12:3 says Therefore with joy shall ye draw water out of the wells of salvation. 

Isaiah 44:3 says For I will pour water upon him that is thirsty, and floods upon the dry ground: I will pour my spirit upon thy seed, and my blessing upon thine offspring. 

In biblical times, the belly was seen as where the issues of life flowed out of or in other words, the center of the heart, in John 7:38 Jesus as the scripture hath said...but Jesus doesn't directly quote the scripture, he gives the interpretation of the scripture. He gives the meaning behind the scripture Joel 2:28, that's why in John 7:39 it says that Jesus was speaking about the Holy Ghost. This is also why on the day of Pentecost, the apostle quoted Joel 2:28, which is the same scripture Jesus was referring to in John 7:38.