Michael Holmes writes in “To Be Continued … The Many Endings of the Gospel of Mark” (2001, p. 19):
the church father Hippolytus (c. 170–236) quotes [Mark] 16:17-18
But as far as I can tell, the citation is not found in the major work of the figure(s) known as Hippolytus. For starters, I checked the indices for the critical editions of Refutatio omnium haeresium (Marcovich, 1986), Kommentar zu Daniel (Bonwetsch and Richard, 2000), De antichristo (Norelli, 1987), Contro Noeto (Simonetti, 2000), De David et de Goliath/De Cantico (Garitte, 1965). There is no citation or even allusion to Mark 16:17-18. What’s going on?
It seems the attribution stems from a rather full citation of Mark 16:17-18 in book 8, chapter 1, paragraph 1 of the fourth-century Apostolic Constitutions (=CA) (8.1.1)
Σημεια δε τοις πιστευσασιν ταυτα παρακολουθησει εν τω ονοματι μου δαιμονια εκβαλουσιν, γλωσσαις λαλησουσιν καιναις, οφεις αρουσιν καν θανασιμον τι πιωσιν ου μη αυτους βλαψει επι αρρωστους χειρας επιθησουσιν και καλως εξουσιν
This occurs in a brief section on the charismatic gifts comprising the first two chapters of book 8 of CA (8.1-2). It is true that CA immediately after contains a reworking of much smaller work known by the similar name, Apostolic Traditions (8.3-45), attributed to an elusive Hippolytus of Rome (on the basis of the famous statue). But it seems precarious at best to attribute the adjacent chapters (8.1-2) to the same author merely because of their placement. This seems to be D. C. Parker’s reasoning when he writes in The Living Text of the Gospels (1997, p. 133)
Hippolytus quotes from verses 17 and 18, in a fragment of a writing on spiritual gifts that is preserved in Apostolic Constitutions 8.1
So the attribution of the citation of the Markan long ending to Hippolytus rests on the placement of anonymous material besides a doubtful attribution (yes, based on the statue) to a figure whose existence is no less obscure than the works attributed to him.