A.T. Robertson Commentary John 7:38

"He that believeth on me, as the scripture hath said, from within him shall flow rivers of living water." — John 7:38 (ASV)

He that believeth on me (ο πιστευων εις εμε). Nominative absolute as is not uncommon.

The scripture (η γραφη). No precise passage can be quoted, though similar idea in several (Isaiah 55:1; Isaiah 58:11; Zechariah 13:1; Zechariah 14:8; Ezekiel 47:1; Joel 3:18). Chrysostom confines it to Isa 28:16 by punctuation (only the nominative absolute as the Scripture).

Out of his belly shall flow rivers of living water (ποταμο εκ της κοιλιας αυτου ρευσουσιν υδατος ζωντος). Some ancient Western writers connect πινετω of verse 37 with ο πιστευων in verse 38. By this arrangement αυτου (his) with κοιλιας is made to refer to Christ, not to the believer. Burney argues that κοιλια is a mistranslation of the Aramaic (fountain, not belly) and that the reference is to Eze 47:1. C.C. Torrey refers to Zec 14:8. But the Eastern writers refer αυτου (his) to the believer who not only quenches in Christ his own thirst, but becomes a source of new streams for others (John 4:14). It is a difficult question and Westcott finally changed his view and held αυτου to refer to Christ. Ρευσουσιν is future active indicative of ρεω, old verb, to flow, here only in the N.T.