Adam Clarke on John 7:38

He that believeth on me, as the Scripture hath said - He who receives me as the Messiah, according to what the Scripture has said concerning me; my person, birth, conduct, preaching, and miracles, being compared with what is written there as ascertaining the true Messiah. Out of his belly - from his heart and soul; for in his soul shall this Spirit dwell.

Living water - As a true spring is ever supplied with water from the great deep, with which it has communication, so shall the soul of the genuine believer be supplied with light, life, love, and liberty, and all the other graces of the indwelling Spirit, from the indwelling Christ. The Jews frequently compare the gifts and influences of the Holy Spirit to water in general - to rain, fountains, wells, rivers, etc., etc. The Scriptures abound in this metaphor. Psa 36:8, Psa 36:9; Isa 44:3, Isa 44:4; Joe 2:23.