A.T. Robertson Commentary Revelation 7:13

"And one of the elders answered, saying unto me, These that are arrayed in white robes, who are they, and whence came they?" — Revelation 7:13 (ASV)

Answered (απεκριθη). First aorist passive (deponent) of αποκρινομα with λεγων (saying), a common (only here in the Apocalypse) Hebrew redundancy in the Gospels . An elder intervenes, though no question has been asked to interpret the vision (Swete).

These (ουτο). Prophetic predicate nominative put before τινες εισιν (who are they). Note article repeated with στολας pointing to verse 9, and accusative also retained after περιβεβλημενο as there. Both "who" and "whence" as in Jos 9:8.