[dangus]

The actual answer is very uncomfortable for those with a post-reformation view of canons is, as well as anyone who believes in sola scriptura:

The passage, cited as scripture by Christ, is from a book which did not make it into the Septuagint, and therefore into the Catholic bible. Nor did it make it past the Council of Jamnia among the Jews (although I think it’s in the Talmud). Problem is I forgot which of the non-canonical books it’s from. An easier non-canonical scripture passage is found in Jude, wherein Jude cites the Book of Enoch.

[maryz]

The passage, cited as scripture by Christ, is from a book which did not make it into the Septuagint

How did you ever know that?! (If you recall which book, please let us know!)

[annalex]

The passage, cited as scripture by Christ, is from a book which did not make it into the Septuagint

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?

(Proverbs 5)

If you find out which book is the quote from, please tell us.