That's what I suspected. St. John the Chrysostom suggested that perhaps the passage should be read in a way so that it is the faith that should be scriptural. St. Jerome suggests something in Proverbs.
CHRYS. But where here does the Scripture say this? No where. What then? We should read, He that believes in Me, as said the Scripture, putting the stop here; and then, out of his belly shall flow rivers of living water: the meaning being, that that was a right kind of belief, which was formed on the evidence of Scripture, not of miracles. Search the Scriptures, he had said before.
JEROME. Or this testimony is taken from the Proverbs, where it is said, Let your fountains be dispersed abroad, and rivers of waters in the streets.
Catena Aurea John 7
The passage in Proverbs has little to do with becoming a fountain for others:
15 Drink water out of thy own cistern, and the streams of thy own well: 16 Let thy fountains be conveyed abroad, and in the streets divide thy waters. 17 Keep them to thyself alone, neither let strangers be partakers with thee. 18 Let thy vein be blessed, and rejoice with the wife of thy youth: 19 Let her be thy dearest hind, and most agreeable fawn: let her breasts inebriate thee at all times; he thou delighted continually with her love. 20 Why art thou seduced, my son, by a strange woman, and art cherished in the bosom of another?If you find out which book is the quote from, please tell us.(Proverbs 5)
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