Who said "A fool couldn't stay this quiet."?

Demaratus (8th–4th century BC). Source: Plutarch, Sayings of Spartans, Demaratus 4 (220a).

What is the original Greek?

ἀλλʼ ὅ γε μωρὸς οὐκ ἂν δύναιτο σιγᾶν ἔφη. Translated from the original Greek by Spartan Surfers.

A fool couldn't stay this quiet.

Demaratus, Plutarch, Sayings of Spartans, Demaratus 4 (220a)

The source

"A fool couldn't stay this quiet." Demaratus said it in Plutarch, Sayings of Spartans, Demaratus 4 (220a). Translated from the original Greek by Spartan Surfers.

"ἀλλʼ ὅ γε μωρὸς οὐκ ἂν δύναιτο σιγᾶν ἔφη."

Original Greek, Plutarch, Sayings of Spartans, Demaratus 4 (220a). Greek: Bernardakis 1889 (Teubner) via Perseus.

Who said it

The Spartans (8th–4th century BC). Laconic sayings from Lycurgus, Leonidas, Dienekes and the mothers of Sparta, recorded by Herodotus and Plutarch.

Questions

Who said "A fool couldn't stay this quiet."?: Demaratus (8th–4th century BC). Source: Plutarch, Sayings of Spartans, Demaratus 4 (220a).
What is the original Greek?: ἀλλʼ ὅ γε μωρὸς οὐκ ἂν δύναιτο σιγᾶν ἔφη. Translated from the original Greek by Spartan Surfers.