ὕδει

From LSJ

πολλοὶ γάρ εἰσιν κλητοὶ ὀλίγοι δὲ ἐκλεκτοί → many are called, but few are chosen

Source

Greek Monotonic

ὕδει: Επικ. αντί ὕδατι, δοτ. του ὕδωρ.